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Theorem List for Metamath Proof Explorer - 2401-2500   *Has distinct variable group(s)
TypeLabelDescription
Statement
 
Theoremcbvexv 2401* Rule used to change bound variables, using implicit substitution. Usage of this theorem is discouraged because it depends on ax-13 2372. See cbvexvw 2040 for a version requiring fewer axioms, to be preferred when sufficient. (Contributed by NM, 21-Jun-1993.) Remove dependency on ax-10 2137, shorten. (Revised by Wolf Lammen, 11-Sep-2023.) (New usage is discouraged.)
(𝑥 = 𝑦 → (𝜑𝜓))       (∃𝑥𝜑 ↔ ∃𝑦𝜓)
 
Theoremcbv1 2402 Rule used to change bound variables, using implicit substitution. Usage of this theorem is discouraged because it depends on ax-13 2372. See cbv1v 2333 with disjoint variable conditions, not depending on ax-13 2372. (Contributed by NM, 5-Aug-1993.) (Revised by Mario Carneiro, 3-Oct-2016.) Format hypotheses to common style. (Revised by Wolf Lammen, 13-May-2018.) (New usage is discouraged.)
𝑥𝜑    &   𝑦𝜑    &   (𝜑 → Ⅎ𝑦𝜓)    &   (𝜑 → Ⅎ𝑥𝜒)    &   (𝜑 → (𝑥 = 𝑦 → (𝜓𝜒)))       (𝜑 → (∀𝑥𝜓 → ∀𝑦𝜒))
 
Theoremcbv2 2403 Rule used to change bound variables, using implicit substitution. Usage of this theorem is discouraged because it depends on ax-13 2372. See cbv2w 2334 with disjoint variable conditions, not depending on ax-13 2372. (Contributed by NM, 5-Aug-1993.) (Revised by Mario Carneiro, 3-Oct-2016.) Format hypotheses to common style, avoid ax-10 2137. (Revised by Wolf Lammen, 10-Sep-2023.) (New usage is discouraged.)
𝑥𝜑    &   𝑦𝜑    &   (𝜑 → Ⅎ𝑦𝜓)    &   (𝜑 → Ⅎ𝑥𝜒)    &   (𝜑 → (𝑥 = 𝑦 → (𝜓𝜒)))       (𝜑 → (∀𝑥𝜓 ↔ ∀𝑦𝜒))
 
Theoremcbv3h 2404 Rule used to change bound variables, using implicit substitution. Usage of this theorem is discouraged because it depends on ax-13 2372. Use the weaker cbv3hv 2337 if possible. (Contributed by NM, 8-Jun-1993.) (Proof shortened by Andrew Salmon, 25-May-2011.) (Proof shortened by Wolf Lammen, 12-May-2018.) (New usage is discouraged.)
(𝜑 → ∀𝑦𝜑)    &   (𝜓 → ∀𝑥𝜓)    &   (𝑥 = 𝑦 → (𝜑𝜓))       (∀𝑥𝜑 → ∀𝑦𝜓)
 
Theoremcbv1h 2405 Rule used to change bound variables, using implicit substitution. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by NM, 11-May-1993.) (Proof shortened by Wolf Lammen, 13-May-2018.) (New usage is discouraged.)
(𝜑 → (𝜓 → ∀𝑦𝜓))    &   (𝜑 → (𝜒 → ∀𝑥𝜒))    &   (𝜑 → (𝑥 = 𝑦 → (𝜓𝜒)))       (∀𝑥𝑦𝜑 → (∀𝑥𝜓 → ∀𝑦𝜒))
 
Theoremcbv2h 2406 Rule used to change bound variables, using implicit substitution. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by NM, 11-May-1993.) (New usage is discouraged.)
(𝜑 → (𝜓 → ∀𝑦𝜓))    &   (𝜑 → (𝜒 → ∀𝑥𝜒))    &   (𝜑 → (𝑥 = 𝑦 → (𝜓𝜒)))       (∀𝑥𝑦𝜑 → (∀𝑥𝜓 ↔ ∀𝑦𝜒))
 
Theoremcbvald 2407* Deduction used to change bound variables, using implicit substitution, particularly useful in conjunction with dvelim 2451. Usage of this theorem is discouraged because it depends on ax-13 2372. See cbvaldw 2335 for a version with 𝑥, 𝑦 disjoint, not depending on ax-13 2372. (Contributed by NM, 2-Jan-2002.) (Revised by Mario Carneiro, 6-Oct-2016.) (Revised by Wolf Lammen, 13-May-2018.) (New usage is discouraged.)
𝑦𝜑    &   (𝜑 → Ⅎ𝑦𝜓)    &   (𝜑 → (𝑥 = 𝑦 → (𝜓𝜒)))       (𝜑 → (∀𝑥𝜓 ↔ ∀𝑦𝜒))
 
Theoremcbvexd 2408* Deduction used to change bound variables, using implicit substitution, particularly useful in conjunction with dvelim 2451. Usage of this theorem is discouraged because it depends on ax-13 2372. Use the weaker cbvexdw 2336 if possible. (Contributed by NM, 2-Jan-2002.) (Revised by Mario Carneiro, 6-Oct-2016.) (New usage is discouraged.)
𝑦𝜑    &   (𝜑 → Ⅎ𝑦𝜓)    &   (𝜑 → (𝑥 = 𝑦 → (𝜓𝜒)))       (𝜑 → (∃𝑥𝜓 ↔ ∃𝑦𝜒))
 
Theoremcbvaldva 2409* Rule used to change the bound variable in a universal quantifier with implicit substitution. Deduction form. Usage of this theorem is discouraged because it depends on ax-13 2372. Use the weaker cbvaldvaw 2041 if possible. (Contributed by David Moews, 1-May-2017.) (New usage is discouraged.)
((𝜑𝑥 = 𝑦) → (𝜓𝜒))       (𝜑 → (∀𝑥𝜓 ↔ ∀𝑦𝜒))
 
Theoremcbvexdva 2410* Rule used to change the bound variable in an existential quantifier with implicit substitution. Deduction form. Usage of this theorem is discouraged because it depends on ax-13 2372. Use the weaker cbvexdvaw 2042 if possible. (Contributed by David Moews, 1-May-2017.) (New usage is discouraged.)
((𝜑𝑥 = 𝑦) → (𝜓𝜒))       (𝜑 → (∃𝑥𝜓 ↔ ∃𝑦𝜒))
 
Theoremcbval2 2411* Rule used to change bound variables, using implicit substitution. Usage of this theorem is discouraged because it depends on ax-13 2372. Use the weaker cbval2v 2340 if possible. (Contributed by NM, 22-Dec-2003.) (Revised by Mario Carneiro, 6-Oct-2016.) (Proof shortened by Wolf Lammen, 11-Sep-2023.) (New usage is discouraged.)
𝑧𝜑    &   𝑤𝜑    &   𝑥𝜓    &   𝑦𝜓    &   ((𝑥 = 𝑧𝑦 = 𝑤) → (𝜑𝜓))       (∀𝑥𝑦𝜑 ↔ ∀𝑧𝑤𝜓)
 
Theoremcbvex2 2412* Rule used to change bound variables, using implicit substitution. Usage of this theorem is discouraged because it depends on ax-13 2372. Use the weaker cbvex2v 2342 if possible. (Contributed by NM, 14-Sep-2003.) (Revised by Mario Carneiro, 6-Oct-2016.) (Proof shortened by Wolf Lammen, 16-Jun-2019.) (New usage is discouraged.)
𝑧𝜑    &   𝑤𝜑    &   𝑥𝜓    &   𝑦𝜓    &   ((𝑥 = 𝑧𝑦 = 𝑤) → (𝜑𝜓))       (∃𝑥𝑦𝜑 ↔ ∃𝑧𝑤𝜓)
 
Theoremcbval2vv 2413* Rule used to change bound variables, using implicit substitution. Usage of this theorem is discouraged because it depends on ax-13 2372. Use the weaker cbval2vw 2043 if possible. (Contributed by NM, 4-Feb-2005.) Remove dependency on ax-10 2137. (Revised by Wolf Lammen, 18-Jul-2021.) (New usage is discouraged.)
((𝑥 = 𝑧𝑦 = 𝑤) → (𝜑𝜓))       (∀𝑥𝑦𝜑 ↔ ∀𝑧𝑤𝜓)
 
Theoremcbvex2vv 2414* Rule used to change bound variables, using implicit substitution. Usage of this theorem is discouraged because it depends on ax-13 2372. Use the weaker cbvex2vw 2044 if possible. (Contributed by NM, 26-Jul-1995.) Remove dependency on ax-10 2137. (Revised by Wolf Lammen, 18-Jul-2021.) (New usage is discouraged.)
((𝑥 = 𝑧𝑦 = 𝑤) → (𝜑𝜓))       (∃𝑥𝑦𝜑 ↔ ∃𝑧𝑤𝜓)
 
Theoremcbvex4v 2415* Rule used to change bound variables, using implicit substitution. Usage of this theorem is discouraged because it depends on ax-13 2372. Use the weaker cbvex4vw 2045 if possible. (Contributed by NM, 26-Jul-1995.) (New usage is discouraged.)
((𝑥 = 𝑣𝑦 = 𝑢) → (𝜑𝜓))    &   ((𝑧 = 𝑓𝑤 = 𝑔) → (𝜓𝜒))       (∃𝑥𝑦𝑧𝑤𝜑 ↔ ∃𝑣𝑢𝑓𝑔𝜒)
 
Theoremequs4 2416 Lemma used in proofs of implicit substitution properties. The converse requires either a disjoint variable condition (sbalex 2235) or a nonfreeness hypothesis (equs45f 2459). Usage of this theorem is discouraged because it depends on ax-13 2372. See equs4v 2003 for a weaker version requiring fewer axioms. (Contributed by NM, 10-May-1993.) (Proof shortened by Mario Carneiro, 20-May-2014.) (Proof shortened by Wolf Lammen, 5-Feb-2018.) (New usage is discouraged.)
(∀𝑥(𝑥 = 𝑦𝜑) → ∃𝑥(𝑥 = 𝑦𝜑))
 
Theoremequsal 2417 An equivalence related to implicit substitution. Usage of this theorem is discouraged because it depends on ax-13 2372. See equsalvw 2007 and equsalv 2259 for versions with disjoint variable conditions proved from fewer axioms. See also the dual form equsex 2418. (Contributed by NM, 2-Jun-1993.) (Proof shortened by Andrew Salmon, 12-Aug-2011.) (Revised by Mario Carneiro, 3-Oct-2016.) (Proof shortened by Wolf Lammen, 5-Feb-2018.) (New usage is discouraged.)
𝑥𝜓    &   (𝑥 = 𝑦 → (𝜑𝜓))       (∀𝑥(𝑥 = 𝑦𝜑) ↔ 𝜓)
 
Theoremequsex 2418 An equivalence related to implicit substitution. Usage of this theorem is discouraged because it depends on ax-13 2372. See equsexvw 2008 and equsexv 2260 for versions with disjoint variable conditions proved from fewer axioms. See also the dual form equsal 2417. See equsexALT 2419 for an alternate proof. (Contributed by NM, 5-Aug-1993.) (Revised by Mario Carneiro, 3-Oct-2016.) (Proof shortened by Wolf Lammen, 6-Feb-2018.) (New usage is discouraged.)
𝑥𝜓    &   (𝑥 = 𝑦 → (𝜑𝜓))       (∃𝑥(𝑥 = 𝑦𝜑) ↔ 𝜓)
 
TheoremequsexALT 2419 Alternate proof of equsex 2418. This proves the result directly, instead of as a corollary of equsal 2417 via equs4 2416. Note in particular that only existential quantifiers appear in the proof and that the only step requiring ax-13 2372 is ax6e 2383. This proof mimics that of equsal 2417 (in particular, note that pm5.32i 575, exbii 1850, 19.41 2228, mpbiran 706 correspond respectively to pm5.74i 270, albii 1822, 19.23 2204, a1bi 363). (Contributed by BJ, 20-Aug-2020.) (Proof modification is discouraged.) (New usage is discouraged.)
𝑥𝜓    &   (𝑥 = 𝑦 → (𝜑𝜓))       (∃𝑥(𝑥 = 𝑦𝜑) ↔ 𝜓)
 
Theoremequsalh 2420 An equivalence related to implicit substitution. Usage of this theorem is discouraged because it depends on ax-13 2372. See equsalhw 2288 for a version with a disjoint variable condition requiring fewer axioms. (Contributed by NM, 2-Jun-1993.) (New usage is discouraged.)
(𝜓 → ∀𝑥𝜓)    &   (𝑥 = 𝑦 → (𝜑𝜓))       (∀𝑥(𝑥 = 𝑦𝜑) ↔ 𝜓)
 
Theoremequsexh 2421 An equivalence related to implicit substitution. Usage of this theorem is discouraged because it depends on ax-13 2372. See equsexhv 2289 for a version with a disjoint variable condition which does not require ax-13 2372. (Contributed by NM, 5-Aug-1993.) (New usage is discouraged.)
(𝜓 → ∀𝑥𝜓)    &   (𝑥 = 𝑦 → (𝜑𝜓))       (∃𝑥(𝑥 = 𝑦𝜑) ↔ 𝜓)
 
Theoremaxc15 2422 Derivation of set.mm's original ax-c15 36903 from ax-c11n 36902 and the shorter ax-12 2171 that has replaced it.

Theorem ax12 2423 shows the reverse derivation of ax-12 2171 from ax-c15 36903.

Normally, axc15 2422 should be used rather than ax-c15 36903, except by theorems specifically studying the latter's properties. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by NM, 2-Feb-2007.) (Proof shortened by Wolf Lammen, 26-Mar-2023.) (New usage is discouraged.)

(¬ ∀𝑥 𝑥 = 𝑦 → (𝑥 = 𝑦 → (𝜑 → ∀𝑥(𝑥 = 𝑦𝜑))))
 
Theoremax12 2423 Rederivation of Axiom ax-12 2171 from ax12v 2172 (used only via sp 2176) , axc11r 2366, and axc15 2422 (on top of Tarski's FOL). Since this version depends on ax-13 2372, usage of the weaker ax12v 2172, ax12w 2129, ax12i 1970 are preferred. (Contributed by NM, 22-Jan-2007.) Proof uses contemporary axioms. (Revised by Wolf Lammen, 8-Aug-2020.) (Proof shortened by BJ, 4-Jul-2021.) (New usage is discouraged.)
(𝑥 = 𝑦 → (∀𝑦𝜑 → ∀𝑥(𝑥 = 𝑦𝜑)))
 
Theoremax12b 2424 A bidirectional version of axc15 2422. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by NM, 30-Jun-2006.) (New usage is discouraged.)
((¬ ∀𝑥 𝑥 = 𝑦𝑥 = 𝑦) → (𝜑 ↔ ∀𝑥(𝑥 = 𝑦𝜑)))
 
Theoremax13ALT 2425 Alternate proof of ax13 2375 from FOL, sp 2176, and axc9 2382. (Contributed by NM, 21-Dec-2015.) (Proof shortened by Wolf Lammen, 31-Jan-2018.) (Proof modification is discouraged.) (New usage is discouraged.)
𝑥 = 𝑦 → (𝑦 = 𝑧 → ∀𝑥 𝑦 = 𝑧))
 
Theoremaxc11n 2426 Derive set.mm's original ax-c11n 36902 from others. Commutation law for identical variable specifiers. The antecedent and consequent are true when 𝑥 and 𝑦 are substituted with the same variable. Lemma L12 in [Megill] p. 445 (p. 12 of the preprint). If a disjoint variable condition is added on 𝑥 and 𝑦, then this becomes an instance of aevlem 2058. Use aecom 2427 instead when this does not lengthen the proof. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by NM, 10-May-1993.) (Revised by NM, 7-Nov-2015.) (Proof shortened by Wolf Lammen, 6-Mar-2018.) (Revised by Wolf Lammen, 30-Nov-2019.) (Proof shortened by BJ, 29-Mar-2021.) (Proof shortened by Wolf Lammen, 2-Jul-2021.) (New usage is discouraged.)
(∀𝑥 𝑥 = 𝑦 → ∀𝑦 𝑦 = 𝑥)
 
Theoremaecom 2427 Commutation law for identical variable specifiers. Both sides of the biconditional are true when 𝑥 and 𝑦 are substituted with the same variable. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by NM, 10-May-1993.) Change to a biconditional. (Revised by BJ, 26-Sep-2019.) (New usage is discouraged.)
(∀𝑥 𝑥 = 𝑦 ↔ ∀𝑦 𝑦 = 𝑥)
 
Theoremaecoms 2428 A commutation rule for identical variable specifiers. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by NM, 10-May-1993.) (New usage is discouraged.)
(∀𝑥 𝑥 = 𝑦𝜑)       (∀𝑦 𝑦 = 𝑥𝜑)
 
Theoremnaecoms 2429 A commutation rule for distinct variable specifiers. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by NM, 2-Jan-2002.) (New usage is discouraged.)
(¬ ∀𝑥 𝑥 = 𝑦𝜑)       (¬ ∀𝑦 𝑦 = 𝑥𝜑)
 
Theoremaxc11 2430 Show that ax-c11 36901 can be derived from ax-c11n 36902 in the form of axc11n 2426. Normally, axc11 2430 should be used rather than ax-c11 36901, except by theorems specifically studying the latter's properties. Usage of this theorem is discouraged because it depends on ax-13 2372. Use the weaker axc11v 2256 when possible. (Contributed by NM, 16-May-2008.) (Proof shortened by Wolf Lammen, 21-Apr-2018.) (New usage is discouraged.)
(∀𝑥 𝑥 = 𝑦 → (∀𝑥𝜑 → ∀𝑦𝜑))
 
Theoremhbae 2431 All variables are effectively bound in an identical variable specifier. Usage of this theorem is discouraged because it depends on ax-13 2372. Use the weaker hbaev 2062 when possible. (Contributed by NM, 13-May-1993.) (Proof shortened by Wolf Lammen, 21-Apr-2018.) (New usage is discouraged.)
(∀𝑥 𝑥 = 𝑦 → ∀𝑧𝑥 𝑥 = 𝑦)
 
Theoremhbnae 2432 All variables are effectively bound in a distinct variable specifier. Lemma L19 in [Megill] p. 446 (p. 14 of the preprint). Usage of this theorem is discouraged because it depends on ax-13 2372. Use the weaker hbnaev 2065 when possible. (Contributed by NM, 13-May-1993.) (New usage is discouraged.)
(¬ ∀𝑥 𝑥 = 𝑦 → ∀𝑧 ¬ ∀𝑥 𝑥 = 𝑦)
 
Theoremnfae 2433 All variables are effectively bound in an identical variable specifier. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by Mario Carneiro, 11-Aug-2016.) (New usage is discouraged.)
𝑧𝑥 𝑥 = 𝑦
 
Theoremnfnae 2434 All variables are effectively bound in a distinct variable specifier. Usage of this theorem is discouraged because it depends on ax-13 2372. Use the weaker nfnaew 2145 when possible. (Contributed by Mario Carneiro, 11-Aug-2016.) (New usage is discouraged.)
𝑧 ¬ ∀𝑥 𝑥 = 𝑦
 
Theoremhbnaes 2435 Rule that applies hbnae 2432 to antecedent. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by NM, 15-May-1993.) (New usage is discouraged.)
(∀𝑧 ¬ ∀𝑥 𝑥 = 𝑦𝜑)       (¬ ∀𝑥 𝑥 = 𝑦𝜑)
 
Theoremaxc16i 2436* Inference with axc16 2253 as its conclusion. (Contributed by NM, 20-May-2008.) (Proof modification is discouraged.) Usage of this theorem is discouraged because it depends on ax-13 2372. Use axc16 2253 instead. (New usage is discouraged.)
(𝑥 = 𝑧 → (𝜑𝜓))    &   (𝜓 → ∀𝑥𝜓)       (∀𝑥 𝑥 = 𝑦 → (𝜑 → ∀𝑥𝜑))
 
Theoremaxc16nfALT 2437* Alternate proof of axc16nf 2255, shorter but requiring ax-11 2154 and ax-13 2372. (Contributed by Mario Carneiro, 7-Oct-2016.) (Proof modification is discouraged.) (New usage is discouraged.)
(∀𝑥 𝑥 = 𝑦 → Ⅎ𝑧𝜑)
 
Theoremdral2 2438 Formula-building lemma for use with the Distinctor Reduction Theorem. Part of Theorem 9.4 of [Megill] p. 448 (p. 16 of preprint). Usage of this theorem is discouraged because it depends on ax-13 2372. Usage of albidv 1923 is preferred, which requires fewer axioms. (Contributed by NM, 27-Feb-2005.) Allow a shortening of dral1 2439. (Revised by Wolf Lammen, 4-Mar-2018.) (New usage is discouraged.)
(∀𝑥 𝑥 = 𝑦 → (𝜑𝜓))       (∀𝑥 𝑥 = 𝑦 → (∀𝑧𝜑 ↔ ∀𝑧𝜓))
 
Theoremdral1 2439 Formula-building lemma for use with the Distinctor Reduction Theorem. Part of Theorem 9.4 of [Megill] p. 448 (p. 16 of preprint). Usage of this theorem is discouraged because it depends on ax-13 2372. Use the weaker dral1v 2367 if possible. (Contributed by NM, 24-Nov-1994.) Remove dependency on ax-11 2154. (Revised by Wolf Lammen, 6-Sep-2018.) (New usage is discouraged.)
(∀𝑥 𝑥 = 𝑦 → (𝜑𝜓))       (∀𝑥 𝑥 = 𝑦 → (∀𝑥𝜑 ↔ ∀𝑦𝜓))
 
Theoremdral1ALT 2440 Alternate proof of dral1 2439, shorter but requiring ax-11 2154. (Contributed by NM, 24-Nov-1994.) (Proof shortened by Wolf Lammen, 22-Apr-2018.) (New usage is discouraged.) (Proof modification is discouraged.)
(∀𝑥 𝑥 = 𝑦 → (𝜑𝜓))       (∀𝑥 𝑥 = 𝑦 → (∀𝑥𝜑 ↔ ∀𝑦𝜓))
 
Theoremdrex1 2441 Formula-building lemma for use with the Distinctor Reduction Theorem. Part of Theorem 9.4 of [Megill] p. 448 (p. 16 of preprint). Usage of this theorem is discouraged because it depends on ax-13 2372. Use the weaker drex1v 2369 if possible. (Contributed by NM, 27-Feb-2005.) (New usage is discouraged.)
(∀𝑥 𝑥 = 𝑦 → (𝜑𝜓))       (∀𝑥 𝑥 = 𝑦 → (∃𝑥𝜑 ↔ ∃𝑦𝜓))
 
Theoremdrex2 2442 Formula-building lemma for use with the Distinctor Reduction Theorem. Part of Theorem 9.4 of [Megill] p. 448 (p. 16 of preprint). Usage of this theorem is discouraged because it depends on ax-13 2372. Usage of exbidv 1924 is preferred, which requires fewer axioms. (Contributed by NM, 27-Feb-2005.) (New usage is discouraged.)
(∀𝑥 𝑥 = 𝑦 → (𝜑𝜓))       (∀𝑥 𝑥 = 𝑦 → (∃𝑧𝜑 ↔ ∃𝑧𝜓))
 
Theoremdrnf1 2443 Formula-building lemma for use with the Distinctor Reduction Theorem. (Contributed by Mario Carneiro, 4-Oct-2016.) Usage of this theorem is discouraged because it depends on ax-13 2372. Use drnf1v 2370 instead. (New usage is discouraged.)
(∀𝑥 𝑥 = 𝑦 → (𝜑𝜓))       (∀𝑥 𝑥 = 𝑦 → (Ⅎ𝑥𝜑 ↔ Ⅎ𝑦𝜓))
 
Theoremdrnf2 2444 Formula-building lemma for use with the Distinctor Reduction Theorem. (Contributed by Mario Carneiro, 4-Oct-2016.) (Proof shortened by Wolf Lammen, 5-May-2018.) Usage of this theorem is discouraged because it depends on ax-13 2372. Use nfbidv 1925 instead. (New usage is discouraged.)
(∀𝑥 𝑥 = 𝑦 → (𝜑𝜓))       (∀𝑥 𝑥 = 𝑦 → (Ⅎ𝑧𝜑 ↔ Ⅎ𝑧𝜓))
 
Theoremnfald2 2445 Variation on nfald 2322 which adds the hypothesis that 𝑥 and 𝑦 are distinct in the inner subproof. (Contributed by Mario Carneiro, 8-Oct-2016.) Usage of this theorem is discouraged because it depends on ax-13 2372. Use nfald 2322 instead. (New usage is discouraged.)
𝑦𝜑    &   ((𝜑 ∧ ¬ ∀𝑥 𝑥 = 𝑦) → Ⅎ𝑥𝜓)       (𝜑 → Ⅎ𝑥𝑦𝜓)
 
Theoremnfexd2 2446 Variation on nfexd 2323 which adds the hypothesis that 𝑥 and 𝑦 are distinct in the inner subproof. (Contributed by Mario Carneiro, 8-Oct-2016.) Usage of this theorem is discouraged because it depends on ax-13 2372. Use nfexd 2323 instead. (New usage is discouraged.)
𝑦𝜑    &   ((𝜑 ∧ ¬ ∀𝑥 𝑥 = 𝑦) → Ⅎ𝑥𝜓)       (𝜑 → Ⅎ𝑥𝑦𝜓)
 
Theoremexdistrf 2447 Distribution of existential quantifiers, with a bound-variable hypothesis saying that 𝑦 is not free in 𝜑, but 𝑥 can be free in 𝜑 (and there is no distinct variable condition on 𝑥 and 𝑦). (Contributed by Mario Carneiro, 20-Mar-2013.) (Proof shortened by Wolf Lammen, 14-May-2018.) Usage of this theorem is discouraged because it depends on ax-13 2372. Use exdistr 1958 instead. (New usage is discouraged.)
(¬ ∀𝑥 𝑥 = 𝑦 → Ⅎ𝑦𝜑)       (∃𝑥𝑦(𝜑𝜓) → ∃𝑥(𝜑 ∧ ∃𝑦𝜓))
 
Theoremdvelimf 2448 Version of dvelimv 2452 without any variable restrictions. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by NM, 1-Oct-2002.) (Revised by Mario Carneiro, 6-Oct-2016.) (Proof shortened by Wolf Lammen, 11-May-2018.) (New usage is discouraged.)
𝑥𝜑    &   𝑧𝜓    &   (𝑧 = 𝑦 → (𝜑𝜓))       (¬ ∀𝑥 𝑥 = 𝑦 → Ⅎ𝑥𝜓)
 
Theoremdvelimdf 2449 Deduction form of dvelimf 2448. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by NM, 7-Apr-2004.) (Revised by Mario Carneiro, 6-Oct-2016.) (Proof shortened by Wolf Lammen, 11-May-2018.) (New usage is discouraged.)
𝑥𝜑    &   𝑧𝜑    &   (𝜑 → Ⅎ𝑥𝜓)    &   (𝜑 → Ⅎ𝑧𝜒)    &   (𝜑 → (𝑧 = 𝑦 → (𝜓𝜒)))       (𝜑 → (¬ ∀𝑥 𝑥 = 𝑦 → Ⅎ𝑥𝜒))
 
Theoremdvelimh 2450 Version of dvelim 2451 without any variable restrictions. Usage of this theorem is discouraged because it depends on ax-13 2372. Check out dvelimhw 2343 for a version requiring fewer axioms. (Contributed by NM, 1-Oct-2002.) (Proof shortened by Wolf Lammen, 11-May-2018.) (New usage is discouraged.)
(𝜑 → ∀𝑥𝜑)    &   (𝜓 → ∀𝑧𝜓)    &   (𝑧 = 𝑦 → (𝜑𝜓))       (¬ ∀𝑥 𝑥 = 𝑦 → (𝜓 → ∀𝑥𝜓))
 
Theoremdvelim 2451* This theorem can be used to eliminate a distinct variable restriction on 𝑥 and 𝑧 and replace it with the "distinctor" ¬ ∀𝑥𝑥 = 𝑦 as an antecedent. 𝜑 normally has 𝑧 free and can be read 𝜑(𝑧), and 𝜓 substitutes 𝑦 for 𝑧 and can be read 𝜑(𝑦). We do not require that 𝑥 and 𝑦 be distinct: if they are not, the distinctor will become false (in multiple-element domains of discourse) and "protect" the consequent.

To obtain a closed-theorem form of this inference, prefix the hypotheses with 𝑥𝑧, conjoin them, and apply dvelimdf 2449.

Other variants of this theorem are dvelimh 2450 (with no distinct variable restrictions) and dvelimhw 2343 (that avoids ax-13 2372). Usage of this theorem is discouraged because it depends on ax-13 2372. Check out dvelimhw 2343 for a version requiring fewer axioms. (Contributed by NM, 23-Nov-1994.) (New usage is discouraged.)

(𝜑 → ∀𝑥𝜑)    &   (𝑧 = 𝑦 → (𝜑𝜓))       (¬ ∀𝑥 𝑥 = 𝑦 → (𝜓 → ∀𝑥𝜓))
 
Theoremdvelimv 2452* Similar to dvelim 2451 with first hypothesis replaced by a distinct variable condition. Usage of this theorem is discouraged because it depends on ax-13 2372. Check out dvelimhw 2343 for a version requiring fewer axioms. (Contributed by NM, 25-Jul-2015.) (Proof shortened by Wolf Lammen, 30-Apr-2018.) (New usage is discouraged.)
(𝑧 = 𝑦 → (𝜑𝜓))       (¬ ∀𝑥 𝑥 = 𝑦 → (𝜓 → ∀𝑥𝜓))
 
Theoremdvelimnf 2453* Version of dvelim 2451 using "not free" notation. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by Mario Carneiro, 9-Oct-2016.) (New usage is discouraged.)
𝑥𝜑    &   (𝑧 = 𝑦 → (𝜑𝜓))       (¬ ∀𝑥 𝑥 = 𝑦 → Ⅎ𝑥𝜓)
 
Theoremdveeq2ALT 2454* Alternate proof of dveeq2 2378, shorter but requiring ax-11 2154. (Contributed by NM, 2-Jan-2002.) (Revised by NM, 20-Jul-2015.) (New usage is discouraged.) (Proof modification is discouraged.)
(¬ ∀𝑥 𝑥 = 𝑦 → (𝑧 = 𝑦 → ∀𝑥 𝑧 = 𝑦))
 
Theoremequvini 2455 A variable introduction law for equality. Lemma 15 of [Monk2] p. 109, however we do not require 𝑧 to be distinct from 𝑥 and 𝑦. Usage of this theorem is discouraged because it depends on ax-13 2372. See equvinv 2032 for a shorter proof requiring fewer axioms when 𝑧 is required to be distinct from 𝑥 and 𝑦. (Contributed by NM, 10-Jan-1993.) (Proof shortened by Andrew Salmon, 25-May-2011.) (Proof shortened by Wolf Lammen, 16-Sep-2023.) (New usage is discouraged.)
(𝑥 = 𝑦 → ∃𝑧(𝑥 = 𝑧𝑧 = 𝑦))
 
Theoremequvel 2456 A variable elimination law for equality with no distinct variable requirements. Compare equvini 2455. Usage of this theorem is discouraged because it depends on ax-13 2372. Use equvelv 2034 when possible. (Contributed by NM, 1-Mar-2013.) (Proof shortened by Mario Carneiro, 17-Oct-2016.) (Proof shortened by Wolf Lammen, 15-Jun-2019.) (New usage is discouraged.)
(∀𝑧(𝑧 = 𝑥𝑧 = 𝑦) → 𝑥 = 𝑦)
 
Theoremequs5a 2457 A property related to substitution that unlike equs5 2460 does not require a distinctor antecedent. Usage of this theorem is discouraged because it depends on ax-13 2372. This proof uses ax12 2423, see equs5aALT 2364 for an alternative one using ax-12 2171 but not ax13 2375. Usage of the weaker equs5av 2271 is preferred, which uses ax12v2 2173, but not ax-13 2372. (Contributed by NM, 2-Feb-2007.) (New usage is discouraged.)
(∃𝑥(𝑥 = 𝑦 ∧ ∀𝑦𝜑) → ∀𝑥(𝑥 = 𝑦𝜑))
 
Theoremequs5e 2458 A property related to substitution that unlike equs5 2460 does not require a distinctor antecedent. This proof uses ax12 2423, see equs5eALT 2365 for an alternative one using ax-12 2171 but not ax13 2375. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by NM, 2-Feb-2007.) (Proof shortened by Wolf Lammen, 15-Jan-2018.) (New usage is discouraged.)
(∃𝑥(𝑥 = 𝑦𝜑) → ∀𝑥(𝑥 = 𝑦 → ∃𝑦𝜑))
 
Theoremequs45f 2459 Two ways of expressing substitution when 𝑦 is not free in 𝜑. The implication "to the left" is equs4 2416 and does not require the nonfreeness hypothesis. Theorem sbalex 2235 replaces the nonfreeness hypothesis with a disjoint variable condition and equs5 2460 replaces it with a distinctor antecedent. (Contributed by NM, 25-Apr-2008.) (Revised by Mario Carneiro, 4-Oct-2016.) Usage of this theorem is discouraged because it depends on ax-13 2372. Use sbalex 2235 instead. (New usage is discouraged.)
𝑦𝜑       (∃𝑥(𝑥 = 𝑦𝜑) ↔ ∀𝑥(𝑥 = 𝑦𝜑))
 
Theoremequs5 2460 Lemma used in proofs of substitution properties. If there is a disjoint variable condition on 𝑥, 𝑦, then sbalex 2235 can be used instead; if 𝑦 is not free in 𝜑, then equs45f 2459 can be used. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by NM, 14-May-1993.) (Revised by BJ, 1-Oct-2018.) (New usage is discouraged.)
(¬ ∀𝑥 𝑥 = 𝑦 → (∃𝑥(𝑥 = 𝑦𝜑) ↔ ∀𝑥(𝑥 = 𝑦𝜑)))
 
Theoremdveel1 2461* Quantifier introduction when one pair of variables is disjoint. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by NM, 2-Jan-2002.) (New usage is discouraged.)
(¬ ∀𝑥 𝑥 = 𝑦 → (𝑦𝑧 → ∀𝑥 𝑦𝑧))
 
Theoremdveel2 2462* Quantifier introduction when one pair of variables is disjoint. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by NM, 2-Jan-2002.) (New usage is discouraged.)
(¬ ∀𝑥 𝑥 = 𝑦 → (𝑧𝑦 → ∀𝑥 𝑧𝑦))
 
Theoremaxc14 2463 Axiom ax-c14 36905 is redundant if we assume ax-5 1913. Remark 9.6 in [Megill] p. 448 (p. 16 of the preprint), regarding axiom scheme C14'.

Note that 𝑤 is a dummy variable introduced in the proof. Its purpose is to satisfy the distinct variable requirements of dveel2 2462 and ax-5 1913. By the end of the proof it has vanished, and the final theorem has no distinct variable requirements. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by NM, 29-Jun-1995.) (Proof modification is discouraged.) (New usage is discouraged.)

(¬ ∀𝑧 𝑧 = 𝑥 → (¬ ∀𝑧 𝑧 = 𝑦 → (𝑥𝑦 → ∀𝑧 𝑥𝑦)))
 
Theoremsb6x 2464 Equivalence involving substitution for a variable not free. Usage of this theorem is discouraged because it depends on ax-13 2372. Usage of sb6 2088 is preferred, which requires fewer axioms. (Contributed by NM, 2-Jun-1993.) (Revised by Mario Carneiro, 4-Oct-2016.) (New usage is discouraged.)
𝑥𝜑       ([𝑦 / 𝑥]𝜑 ↔ ∀𝑥(𝑥 = 𝑦𝜑))
 
Theoremsbequ5 2465 Substitution does not change an identical variable specifier. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by NM, 15-May-1993.) (New usage is discouraged.)
([𝑤 / 𝑧]∀𝑥 𝑥 = 𝑦 ↔ ∀𝑥 𝑥 = 𝑦)
 
Theoremsbequ6 2466 Substitution does not change a distinctor. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by NM, 5-Aug-1993.) (New usage is discouraged.)
([𝑤 / 𝑧] ¬ ∀𝑥 𝑥 = 𝑦 ↔ ¬ ∀𝑥 𝑥 = 𝑦)
 
Theoremsb5rf 2467 Reversed substitution. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by NM, 3-Feb-2005.) (Revised by Mario Carneiro, 6-Oct-2016.) (Proof shortened by Wolf Lammen, 20-Sep-2018.) (New usage is discouraged.)
𝑦𝜑       (𝜑 ↔ ∃𝑦(𝑦 = 𝑥 ∧ [𝑦 / 𝑥]𝜑))
 
Theoremsb6rf 2468 Reversed substitution. For a version requiring disjoint variables, but fewer axioms, see sb6rfv 2355. Usage of this theorem is discouraged because it depends on ax-13 2372. Use the weaker sb6rfv 2355 if possible. (Contributed by NM, 1-Aug-1993.) (Revised by Mario Carneiro, 6-Oct-2016.) (Proof shortened by Wolf Lammen, 21-Sep-2018.) (New usage is discouraged.)
𝑦𝜑       (𝜑 ↔ ∀𝑦(𝑦 = 𝑥 → [𝑦 / 𝑥]𝜑))
 
Theoremax12vALT 2469* Alternate proof of ax12v2 2173, shorter, but depending on more axioms. (Contributed by NM, 5-Aug-1993.) (New usage is discouraged.) (Proof modification is discouraged.)
(𝑥 = 𝑦 → (𝜑 → ∀𝑥(𝑥 = 𝑦𝜑)))
 
Theorem2ax6elem 2470 We can always find values matching 𝑥 and 𝑦, as long as they are represented by distinct variables. This theorem merges two ax6e 2383 instances 𝑧𝑧 = 𝑥 and 𝑤𝑤 = 𝑦 into a common expression. Alan Sare contributed a variant of this theorem with distinct variable conditions before, see ax6e2nd 42178. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by Wolf Lammen, 27-Sep-2018.) (New usage is discouraged.)
(¬ ∀𝑤 𝑤 = 𝑧 → ∃𝑧𝑤(𝑧 = 𝑥𝑤 = 𝑦))
 
Theorem2ax6e 2471* We can always find values matching 𝑥 and 𝑦, as long as they are represented by distinct variables. Version of 2ax6elem 2470 with a distinct variable constraint. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by Wolf Lammen, 28-Sep-2018.) (Proof shortened by Wolf Lammen, 3-Oct-2023.) (New usage is discouraged.)
𝑧𝑤(𝑧 = 𝑥𝑤 = 𝑦)
 
Theorem2sb5rf 2472* Reversed double substitution. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by NM, 3-Feb-2005.) (Revised by Mario Carneiro, 6-Oct-2016.) Remove distinct variable constraints. (Revised by Wolf Lammen, 28-Sep-2018.) (New usage is discouraged.)
𝑧𝜑    &   𝑤𝜑       (𝜑 ↔ ∃𝑧𝑤((𝑧 = 𝑥𝑤 = 𝑦) ∧ [𝑧 / 𝑥][𝑤 / 𝑦]𝜑))
 
Theorem2sb6rf 2473* Reversed double substitution. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by NM, 3-Feb-2005.) (Revised by Mario Carneiro, 6-Oct-2016.) Remove variable constraints. (Revised by Wolf Lammen, 28-Sep-2018.) (Proof shortened by Wolf Lammen, 13-Apr-2023.) (New usage is discouraged.)
𝑧𝜑    &   𝑤𝜑       (𝜑 ↔ ∀𝑧𝑤((𝑧 = 𝑥𝑤 = 𝑦) → [𝑧 / 𝑥][𝑤 / 𝑦]𝜑))
 
Theoremsbel2x 2474* Elimination of double substitution. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by NM, 5-Aug-1993.) (Proof shortened by Wolf Lammen, 29-Sep-2018.) (New usage is discouraged.)
(𝜑 ↔ ∃𝑥𝑦((𝑥 = 𝑧𝑦 = 𝑤) ∧ [𝑦 / 𝑤][𝑥 / 𝑧]𝜑))
 
Theoremsb4b 2475 Simplified definition of substitution when variables are distinct. Version of sb6 2088 with a distinctor antecedent. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by NM, 27-May-1997.) Revise df-sb 2068. (Revised by Wolf Lammen, 21-Feb-2024.) (New usage is discouraged.)
(¬ ∀𝑥 𝑥 = 𝑡 → ([𝑡 / 𝑥]𝜑 ↔ ∀𝑥(𝑥 = 𝑡𝜑)))
 
Theoremsb4bOLD 2476 Obsolete version of sb4b 2475 as of 21-Feb-2024. (Contributed by NM, 27-May-1997.) Revise df-sb 2068. (Revised by Wolf Lammen, 25-Jul-2023.) (New usage is discouraged.) (Proof modification is discouraged.)
(¬ ∀𝑥 𝑥 = 𝑡 → ([𝑡 / 𝑥]𝜑 ↔ ∀𝑥(𝑥 = 𝑡𝜑)))
 
Theoremsb3b 2477 Simplified definition of substitution when variables are distinct. This is the biconditional strengthening of sb3 2478. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by BJ, 6-Oct-2018.) Shorten sb3 2478. (Revised by Wolf Lammen, 21-Feb-2021.) (New usage is discouraged.)
(¬ ∀𝑥 𝑥 = 𝑦 → ([𝑦 / 𝑥]𝜑 ↔ ∃𝑥(𝑥 = 𝑦𝜑)))
 
Theoremsb3 2478 One direction of a simplified definition of substitution when variables are distinct. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by NM, 5-Aug-1993.) (Proof shortened by Wolf Lammen, 21-Feb-2024.) (New usage is discouraged.)
(¬ ∀𝑥 𝑥 = 𝑦 → (∃𝑥(𝑥 = 𝑦𝜑) → [𝑦 / 𝑥]𝜑))
 
Theoremsb1 2479 One direction of a simplified definition of substitution. The converse requires either a disjoint variable condition (sb5 2268) or a nonfreeness hypothesis (sb5f 2502). Usage of this theorem is discouraged because it depends on ax-13 2372. Use the weaker sb1v 2090 when possible. (Contributed by NM, 13-May-1993.) Revise df-sb 2068. (Revised by Wolf Lammen, 21-Feb-2024.) (New usage is discouraged.)
([𝑦 / 𝑥]𝜑 → ∃𝑥(𝑥 = 𝑦𝜑))
 
Theoremsb2 2480 One direction of a simplified definition of substitution. The converse requires either a disjoint variable condition (sb6 2088) or a nonfreeness hypothesis (sb6f 2501). Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by NM, 13-May-1993.) Revise df-sb 2068. (Revised by Wolf Lammen, 26-Jul-2023.) (New usage is discouraged.)
(∀𝑥(𝑥 = 𝑦𝜑) → [𝑦 / 𝑥]𝜑)
 
Theoremsb3OLD 2481 Obsolete version of sb3 2478 as of 21-Feb-2024. (Contributed by NM, 5-Aug-1993.) (Proof modification is discouraged.) (New usage is discouraged.)
(¬ ∀𝑥 𝑥 = 𝑦 → (∃𝑥(𝑥 = 𝑦𝜑) → [𝑦 / 𝑥]𝜑))
 
Theoremsb1OLD 2482 Obsolete version of sb1 2479 as of 21-Feb-2024. (Contributed by NM, 13-May-1993.) Revise df-sb 2068. (Revised by Wolf Lammen, 29-Jul-2023.) (Proof modification is discouraged.) (New usage is discouraged.)
([𝑦 / 𝑥]𝜑 → ∃𝑥(𝑥 = 𝑦𝜑))
 
Theoremsb3bOLD 2483 Obsolete version of sb3b 2477 as of 21-Feb-2024. (Contributed by BJ, 6-Oct-2018.) (Proof modification is discouraged.) (New usage is discouraged.)
(¬ ∀𝑥 𝑥 = 𝑦 → ([𝑦 / 𝑥]𝜑 ↔ ∃𝑥(𝑥 = 𝑦𝜑)))
 
Theoremsb4a 2484 A version of one implication of sb4b 2475 that does not require a distinctor antecedent. Usage of this theorem is discouraged because it depends on ax-13 2372. Use the weaker sb4av 2236 when possible. (Contributed by NM, 2-Feb-2007.) Revise df-sb 2068. (Revised by Wolf Lammen, 28-Jul-2023.) (New usage is discouraged.)
([𝑡 / 𝑥]∀𝑡𝜑 → ∀𝑥(𝑥 = 𝑡𝜑))
 
Theoremdfsb1 2485 Alternate definition of substitution. Remark 9.1 in [Megill] p. 447 (p. 15 of the preprint). This was the original definition before df-sb 2068. Note that it does not require dummy variables in its definiens; this is done by having 𝑥 free in the first conjunct and bound in the second. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by BJ, 9-Jul-2023.) Revise df-sb 2068. (Revised by Wolf Lammen, 29-Jul-2023.) (New usage is discouraged.)
([𝑦 / 𝑥]𝜑 ↔ ((𝑥 = 𝑦𝜑) ∧ ∃𝑥(𝑥 = 𝑦𝜑)))
 
Theoremhbsb2 2486 Bound-variable hypothesis builder for substitution. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by NM, 14-May-1993.) (New usage is discouraged.)
(¬ ∀𝑥 𝑥 = 𝑦 → ([𝑦 / 𝑥]𝜑 → ∀𝑥[𝑦 / 𝑥]𝜑))
 
Theoremnfsb2 2487 Bound-variable hypothesis builder for substitution. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by Mario Carneiro, 4-Oct-2016.) (New usage is discouraged.)
(¬ ∀𝑥 𝑥 = 𝑦 → Ⅎ𝑥[𝑦 / 𝑥]𝜑)
 
Theoremhbsb2a 2488 Special case of a bound-variable hypothesis builder for substitution. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by NM, 2-Feb-2007.) (New usage is discouraged.)
([𝑦 / 𝑥]∀𝑦𝜑 → ∀𝑥[𝑦 / 𝑥]𝜑)
 
Theoremsb4e 2489 One direction of a simplified definition of substitution that unlike sb4b 2475 does not require a distinctor antecedent. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by NM, 2-Feb-2007.) (New usage is discouraged.)
([𝑦 / 𝑥]𝜑 → ∀𝑥(𝑥 = 𝑦 → ∃𝑦𝜑))
 
Theoremhbsb2e 2490 Special case of a bound-variable hypothesis builder for substitution. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by NM, 2-Feb-2007.) (New usage is discouraged.)
([𝑦 / 𝑥]𝜑 → ∀𝑥[𝑦 / 𝑥]∃𝑦𝜑)
 
Theoremhbsb3 2491 If 𝑦 is not free in 𝜑, 𝑥 is not free in [𝑦 / 𝑥]𝜑. Usage of this theorem is discouraged because it depends on ax-13 2372. Check out bj-hbsb3v 34997 for a weaker version requiring fewer axioms. (Contributed by NM, 14-May-1993.) (New usage is discouraged.)
(𝜑 → ∀𝑦𝜑)       ([𝑦 / 𝑥]𝜑 → ∀𝑥[𝑦 / 𝑥]𝜑)
 
Theoremnfs1 2492 If 𝑦 is not free in 𝜑, 𝑥 is not free in [𝑦 / 𝑥]𝜑. Usage of this theorem is discouraged because it depends on ax-13 2372. Check out nfs1v 2153 for a version requiring fewer axioms. (Contributed by Mario Carneiro, 11-Aug-2016.) (New usage is discouraged.)
𝑦𝜑       𝑥[𝑦 / 𝑥]𝜑
 
Theoremaxc16ALT 2493* Alternate proof of axc16 2253, shorter but requiring ax-10 2137, ax-11 2154, ax-13 2372 and using df-nf 1787 and df-sb 2068. (Contributed by NM, 17-May-2008.) (Proof modification is discouraged.) (New usage is discouraged.)
(∀𝑥 𝑥 = 𝑦 → (𝜑 → ∀𝑥𝜑))
 
Theoremaxc16gALT 2494* Alternate proof of axc16g 2252 that uses df-sb 2068 and requires ax-10 2137, ax-11 2154, ax-13 2372. (Contributed by NM, 15-May-1993.) (Proof shortened by Andrew Salmon, 25-May-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
(∀𝑥 𝑥 = 𝑦 → (𝜑 → ∀𝑧𝜑))
 
Theoremequsb1 2495 Substitution applied to an atomic wff. Usage of this theorem is discouraged because it depends on ax-13 2372. Use the weaker equsb1v 2103 if possible. (Contributed by NM, 10-May-1993.) (New usage is discouraged.)
[𝑦 / 𝑥]𝑥 = 𝑦
 
Theoremequsb2 2496 Substitution applied to an atomic wff. Usage of this theorem is discouraged because it depends on ax-13 2372. Check out equsb1v 2103 for a version requiring fewer axioms. (Contributed by NM, 10-May-1993.) (New usage is discouraged.)
[𝑦 / 𝑥]𝑦 = 𝑥
 
Theoremdfsb2 2497 An alternate definition of proper substitution that, like dfsb1 2485, mixes free and bound variables to avoid distinct variable requirements. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by NM, 17-Feb-2005.) (New usage is discouraged.)
([𝑦 / 𝑥]𝜑 ↔ ((𝑥 = 𝑦𝜑) ∨ ∀𝑥(𝑥 = 𝑦𝜑)))
 
Theoremdfsb3 2498 An alternate definition of proper substitution df-sb 2068 that uses only primitive connectives (no defined terms) on the right-hand side. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by NM, 6-Mar-2007.) (New usage is discouraged.)
([𝑦 / 𝑥]𝜑 ↔ ((𝑥 = 𝑦 → ¬ 𝜑) → ∀𝑥(𝑥 = 𝑦𝜑)))
 
Theoremdrsb1 2499 Formula-building lemma for use with the Distinctor Reduction Theorem. Part of Theorem 9.4 of [Megill] p. 448 (p. 16 of preprint). Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by NM, 2-Jun-1993.) (New usage is discouraged.)
(∀𝑥 𝑥 = 𝑦 → ([𝑧 / 𝑥]𝜑 ↔ [𝑧 / 𝑦]𝜑))
 
Theoremsb2ae 2500* In the case of two successive substitutions for two always equal variables, the second substitution has no effect. Usage of this theorem is discouraged because it depends on ax-13 2372. (Contributed by BJ and WL, 9-Aug-2023.) (New usage is discouraged.)
(∀𝑥 𝑥 = 𝑦 → ([𝑢 / 𝑥][𝑣 / 𝑦]𝜑 ↔ [𝑣 / 𝑦]𝜑))
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