P. 4 of Theorem List - Metamath Proof Explorer

Theorem List for Metamath Proof Explorer - 301-400   *Has distinct variable group(s)

Type	Label	Description
Statement
Theorem	3bitrri 301	A chained inference from transitive law for logical equivalence. (Contributed by NM, 4-Aug-2006.)
⊢ (𝜑 ↔ 𝜓)    &   ⊢ (𝜓 ↔ 𝜒)    &   ⊢ (𝜒 ↔ 𝜃)    ⇒   ⊢ (𝜃 ↔ 𝜑)
Theorem	3bitr2i 302	A chained inference from transitive law for logical equivalence. (Contributed by NM, 4-Aug-2006.)
⊢ (𝜑 ↔ 𝜓)    &   ⊢ (𝜒 ↔ 𝜓)    &   ⊢ (𝜒 ↔ 𝜃)    ⇒   ⊢ (𝜑 ↔ 𝜃)
Theorem	3bitr2ri 303	A chained inference from transitive law for logical equivalence. (Contributed by NM, 4-Aug-2006.)
⊢ (𝜑 ↔ 𝜓)    &   ⊢ (𝜒 ↔ 𝜓)    &   ⊢ (𝜒 ↔ 𝜃)    ⇒   ⊢ (𝜃 ↔ 𝜑)
Theorem	3bitr3i 304	A chained inference from transitive law for logical equivalence. (Contributed by NM, 19-Aug-1993.)
⊢ (𝜑 ↔ 𝜓)    &   ⊢ (𝜑 ↔ 𝜒)    &   ⊢ (𝜓 ↔ 𝜃)    ⇒   ⊢ (𝜒 ↔ 𝜃)
Theorem	3bitr3ri 305	A chained inference from transitive law for logical equivalence. (Contributed by NM, 21-Jun-1993.)
⊢ (𝜑 ↔ 𝜓)    &   ⊢ (𝜑 ↔ 𝜒)    &   ⊢ (𝜓 ↔ 𝜃)    ⇒   ⊢ (𝜃 ↔ 𝜒)
Theorem	3bitr4i 306	A chained inference from transitive law for logical equivalence. This inference is frequently used to apply a definition to both sides of a logical equivalence. (Contributed by NM, 3-Jan-1993.)
⊢ (𝜑 ↔ 𝜓)    &   ⊢ (𝜒 ↔ 𝜑)    &   ⊢ (𝜃 ↔ 𝜓)    ⇒   ⊢ (𝜒 ↔ 𝜃)
Theorem	3bitr4ri 307	A chained inference from transitive law for logical equivalence. (Contributed by NM, 2-Sep-1995.)
⊢ (𝜑 ↔ 𝜓)    &   ⊢ (𝜒 ↔ 𝜑)    &   ⊢ (𝜃 ↔ 𝜓)    ⇒   ⊢ (𝜃 ↔ 𝜒)
Theorem	3bitrd 308	Deduction from transitivity of biconditional. (Contributed by NM, 13-Aug-1999.)
⊢ (𝜑 → (𝜓 ↔ 𝜒))    &   ⊢ (𝜑 → (𝜒 ↔ 𝜃))    &   ⊢ (𝜑 → (𝜃 ↔ 𝜏))    ⇒   ⊢ (𝜑 → (𝜓 ↔ 𝜏))
Theorem	3bitrrd 309	Deduction from transitivity of biconditional. (Contributed by NM, 4-Aug-2006.)
⊢ (𝜑 → (𝜓 ↔ 𝜒))    &   ⊢ (𝜑 → (𝜒 ↔ 𝜃))    &   ⊢ (𝜑 → (𝜃 ↔ 𝜏))    ⇒   ⊢ (𝜑 → (𝜏 ↔ 𝜓))
Theorem	3bitr2d 310	Deduction from transitivity of biconditional. (Contributed by NM, 4-Aug-2006.)
⊢ (𝜑 → (𝜓 ↔ 𝜒))    &   ⊢ (𝜑 → (𝜃 ↔ 𝜒))    &   ⊢ (𝜑 → (𝜃 ↔ 𝜏))    ⇒   ⊢ (𝜑 → (𝜓 ↔ 𝜏))
Theorem	3bitr2rd 311	Deduction from transitivity of biconditional. (Contributed by NM, 4-Aug-2006.)
⊢ (𝜑 → (𝜓 ↔ 𝜒))    &   ⊢ (𝜑 → (𝜃 ↔ 𝜒))    &   ⊢ (𝜑 → (𝜃 ↔ 𝜏))    ⇒   ⊢ (𝜑 → (𝜏 ↔ 𝜓))
Theorem	3bitr3d 312	Deduction from transitivity of biconditional. Useful for converting conditional definitions in a formula. (Contributed by NM, 24-Apr-1996.)
⊢ (𝜑 → (𝜓 ↔ 𝜒))    &   ⊢ (𝜑 → (𝜓 ↔ 𝜃))    &   ⊢ (𝜑 → (𝜒 ↔ 𝜏))    ⇒   ⊢ (𝜑 → (𝜃 ↔ 𝜏))
Theorem	3bitr3rd 313	Deduction from transitivity of biconditional. (Contributed by NM, 4-Aug-2006.)
⊢ (𝜑 → (𝜓 ↔ 𝜒))    &   ⊢ (𝜑 → (𝜓 ↔ 𝜃))    &   ⊢ (𝜑 → (𝜒 ↔ 𝜏))    ⇒   ⊢ (𝜑 → (𝜏 ↔ 𝜃))
Theorem	3bitr4d 314	Deduction from transitivity of biconditional. Useful for converting conditional definitions in a formula. (Contributed by NM, 18-Oct-1995.)
⊢ (𝜑 → (𝜓 ↔ 𝜒))    &   ⊢ (𝜑 → (𝜃 ↔ 𝜓))    &   ⊢ (𝜑 → (𝜏 ↔ 𝜒))    ⇒   ⊢ (𝜑 → (𝜃 ↔ 𝜏))
Theorem	3bitr4rd 315	Deduction from transitivity of biconditional. (Contributed by NM, 4-Aug-2006.)
⊢ (𝜑 → (𝜓 ↔ 𝜒))    &   ⊢ (𝜑 → (𝜃 ↔ 𝜓))    &   ⊢ (𝜑 → (𝜏 ↔ 𝜒))    ⇒   ⊢ (𝜑 → (𝜏 ↔ 𝜃))
Theorem	3bitr3g 316	More general version of 3bitr3i 304. Useful for converting definitions in a formula. (Contributed by NM, 4-Jun-1995.)
⊢ (𝜑 → (𝜓 ↔ 𝜒))    &   ⊢ (𝜓 ↔ 𝜃)    &   ⊢ (𝜒 ↔ 𝜏)    ⇒   ⊢ (𝜑 → (𝜃 ↔ 𝜏))
Theorem	3bitr4g 317	More general version of 3bitr4i 306. Useful for converting definitions in a formula. (Contributed by NM, 11-May-1993.)
⊢ (𝜑 → (𝜓 ↔ 𝜒))    &   ⊢ (𝜃 ↔ 𝜓)    &   ⊢ (𝜏 ↔ 𝜒)    ⇒   ⊢ (𝜑 → (𝜃 ↔ 𝜏))
Theorem	notnotb 318	Double negation. Theorem *4.13 of [WhiteheadRussell] p. 117. (Contributed by NM, 3-Jan-1993.)
⊢ (𝜑 ↔ ¬ ¬ 𝜑)
Theorem	con34b 319	A biconditional form of contraposition. Theorem *4.1 of [WhiteheadRussell] p. 116. (Contributed by NM, 11-May-1993.)
⊢ ((𝜑 → 𝜓) ↔ (¬ 𝜓 → ¬ 𝜑))
Theorem	con4bid 320	A contraposition deduction. (Contributed by NM, 21-May-1994.)
⊢ (𝜑 → (¬ 𝜓 ↔ ¬ 𝜒))    ⇒   ⊢ (𝜑 → (𝜓 ↔ 𝜒))
Theorem	notbid 321	Deduction negating both sides of a logical equivalence. (Contributed by NM, 21-May-1994.)
⊢ (𝜑 → (𝜓 ↔ 𝜒))    ⇒   ⊢ (𝜑 → (¬ 𝜓 ↔ ¬ 𝜒))
Theorem	notbi 322	Contraposition. Theorem *4.11 of [WhiteheadRussell] p. 117. (Contributed by NM, 21-May-1994.) (Proof shortened by Wolf Lammen, 12-Jun-2013.)
⊢ ((𝜑 ↔ 𝜓) ↔ (¬ 𝜑 ↔ ¬ 𝜓))
Theorem	notbii 323	Negate both sides of a logical equivalence. (Contributed by NM, 3-Jan-1993.) (Proof shortened by Wolf Lammen, 19-May-2013.)
⊢ (𝜑 ↔ 𝜓)    ⇒   ⊢ (¬ 𝜑 ↔ ¬ 𝜓)
Theorem	con4bii 324	A contraposition inference. (Contributed by NM, 21-May-1994.)
⊢ (¬ 𝜑 ↔ ¬ 𝜓)    ⇒   ⊢ (𝜑 ↔ 𝜓)
Theorem	mtbi 325	An inference from a biconditional, related to modus tollens. (Contributed by NM, 15-Nov-1994.) (Proof shortened by Wolf Lammen, 25-Oct-2012.)
⊢ ¬ 𝜑    &   ⊢ (𝜑 ↔ 𝜓)    ⇒   ⊢ ¬ 𝜓
Theorem	mtbir 326	An inference from a biconditional, related to modus tollens. (Contributed by NM, 15-Nov-1994.) (Proof shortened by Wolf Lammen, 14-Oct-2012.)
⊢ ¬ 𝜓    &   ⊢ (𝜑 ↔ 𝜓)    ⇒   ⊢ ¬ 𝜑
Theorem	mtbid 327	A deduction from a biconditional, similar to modus tollens. (Contributed by NM, 26-Nov-1995.)
⊢ (𝜑 → ¬ 𝜓)    &   ⊢ (𝜑 → (𝜓 ↔ 𝜒))    ⇒   ⊢ (𝜑 → ¬ 𝜒)
Theorem	mtbird 328	A deduction from a biconditional, similar to modus tollens. (Contributed by NM, 10-May-1994.)
⊢ (𝜑 → ¬ 𝜒)    &   ⊢ (𝜑 → (𝜓 ↔ 𝜒))    ⇒   ⊢ (𝜑 → ¬ 𝜓)
Theorem	mtbii 329	An inference from a biconditional, similar to modus tollens. (Contributed by NM, 27-Nov-1995.)
⊢ ¬ 𝜓    &   ⊢ (𝜑 → (𝜓 ↔ 𝜒))    ⇒   ⊢ (𝜑 → ¬ 𝜒)
Theorem	mtbiri 330	An inference from a biconditional, similar to modus tollens. (Contributed by NM, 24-Aug-1995.)
⊢ ¬ 𝜒    &   ⊢ (𝜑 → (𝜓 ↔ 𝜒))    ⇒   ⊢ (𝜑 → ¬ 𝜓)
Theorem	sylnib 331	A mixed syllogism inference from an implication and a biconditional. (Contributed by Wolf Lammen, 16-Dec-2013.)
⊢ (𝜑 → ¬ 𝜓)    &   ⊢ (𝜓 ↔ 𝜒)    ⇒   ⊢ (𝜑 → ¬ 𝜒)
Theorem	sylnibr 332	A mixed syllogism inference from an implication and a biconditional. Useful for substituting a consequent with a definition. (Contributed by Wolf Lammen, 16-Dec-2013.)
⊢ (𝜑 → ¬ 𝜓)    &   ⊢ (𝜒 ↔ 𝜓)    ⇒   ⊢ (𝜑 → ¬ 𝜒)
Theorem	sylnbi 333	A mixed syllogism inference from a biconditional and an implication. Useful for substituting an antecedent with a definition. (Contributed by Wolf Lammen, 16-Dec-2013.)
⊢ (𝜑 ↔ 𝜓)    &   ⊢ (¬ 𝜓 → 𝜒)    ⇒   ⊢ (¬ 𝜑 → 𝜒)
Theorem	sylnbir 334	A mixed syllogism inference from a biconditional and an implication. (Contributed by Wolf Lammen, 16-Dec-2013.)
⊢ (𝜓 ↔ 𝜑)    &   ⊢ (¬ 𝜓 → 𝜒)    ⇒   ⊢ (¬ 𝜑 → 𝜒)
Theorem	xchnxbi 335	Replacement of a subexpression by an equivalent one. (Contributed by Wolf Lammen, 27-Sep-2014.)
⊢ (¬ 𝜑 ↔ 𝜓)    &   ⊢ (𝜑 ↔ 𝜒)    ⇒   ⊢ (¬ 𝜒 ↔ 𝜓)
Theorem	xchnxbir 336	Replacement of a subexpression by an equivalent one. (Contributed by Wolf Lammen, 27-Sep-2014.)
⊢ (¬ 𝜑 ↔ 𝜓)    &   ⊢ (𝜒 ↔ 𝜑)    ⇒   ⊢ (¬ 𝜒 ↔ 𝜓)
Theorem	xchbinx 337	Replacement of a subexpression by an equivalent one. (Contributed by Wolf Lammen, 27-Sep-2014.)
⊢ (𝜑 ↔ ¬ 𝜓)    &   ⊢ (𝜓 ↔ 𝜒)    ⇒   ⊢ (𝜑 ↔ ¬ 𝜒)
Theorem	xchbinxr 338	Replacement of a subexpression by an equivalent one. (Contributed by Wolf Lammen, 27-Sep-2014.)
⊢ (𝜑 ↔ ¬ 𝜓)    &   ⊢ (𝜒 ↔ 𝜓)    ⇒   ⊢ (𝜑 ↔ ¬ 𝜒)
Theorem	imbi2i 339	Introduce an antecedent to both sides of a logical equivalence. This and the next three rules are useful for building up wff's around a definition, in order to make use of the definition. (Contributed by NM, 3-Jan-1993.) (Proof shortened by Wolf Lammen, 6-Feb-2013.)
⊢ (𝜑 ↔ 𝜓)    ⇒   ⊢ ((𝜒 → 𝜑) ↔ (𝜒 → 𝜓))
Theorem	jcndOLD 340	Obsolete version of jcnd 166 as of 10-Apr-2024. (Contributed by Glauco Siliprandi, 11-Dec-2019.) (New usage is discouraged.) (Proof modification is discouraged.)
⊢ (𝜑 → 𝜓)    &   ⊢ (𝜑 → ¬ 𝜒)    ⇒   ⊢ (𝜑 → ¬ (𝜓 → 𝜒))
Theorem	bibi2i 341	Inference adding a biconditional to the left in an equivalence. (Contributed by NM, 26-May-1993.) (Proof shortened by Andrew Salmon, 7-May-2011.) (Proof shortened by Wolf Lammen, 16-May-2013.)
⊢ (𝜑 ↔ 𝜓)    ⇒   ⊢ ((𝜒 ↔ 𝜑) ↔ (𝜒 ↔ 𝜓))
Theorem	bibi1i 342	Inference adding a biconditional to the right in an equivalence. (Contributed by NM, 26-May-1993.)
⊢ (𝜑 ↔ 𝜓)    ⇒   ⊢ ((𝜑 ↔ 𝜒) ↔ (𝜓 ↔ 𝜒))
Theorem	bibi12i 343	The equivalence of two equivalences. (Contributed by NM, 26-May-1993.)
⊢ (𝜑 ↔ 𝜓)    &   ⊢ (𝜒 ↔ 𝜃)    ⇒   ⊢ ((𝜑 ↔ 𝜒) ↔ (𝜓 ↔ 𝜃))
Theorem	imbi2d 344	Deduction adding an antecedent to both sides of a logical equivalence. (Contributed by NM, 11-May-1993.)
⊢ (𝜑 → (𝜓 ↔ 𝜒))    ⇒   ⊢ (𝜑 → ((𝜃 → 𝜓) ↔ (𝜃 → 𝜒)))
Theorem	imbi1d 345	Deduction adding a consequent to both sides of a logical equivalence. (Contributed by NM, 11-May-1993.) (Proof shortened by Wolf Lammen, 17-Sep-2013.)
⊢ (𝜑 → (𝜓 ↔ 𝜒))    ⇒   ⊢ (𝜑 → ((𝜓 → 𝜃) ↔ (𝜒 → 𝜃)))
Theorem	bibi2d 346	Deduction adding a biconditional to the left in an equivalence. (Contributed by NM, 11-May-1993.) (Proof shortened by Wolf Lammen, 19-May-2013.)
⊢ (𝜑 → (𝜓 ↔ 𝜒))    ⇒   ⊢ (𝜑 → ((𝜃 ↔ 𝜓) ↔ (𝜃 ↔ 𝜒)))
Theorem	bibi1d 347	Deduction adding a biconditional to the right in an equivalence. (Contributed by NM, 11-May-1993.)
⊢ (𝜑 → (𝜓 ↔ 𝜒))    ⇒   ⊢ (𝜑 → ((𝜓 ↔ 𝜃) ↔ (𝜒 ↔ 𝜃)))
Theorem	imbi12d 348	Deduction joining two equivalences to form equivalence of implications. (Contributed by NM, 16-May-1993.)
⊢ (𝜑 → (𝜓 ↔ 𝜒))    &   ⊢ (𝜑 → (𝜃 ↔ 𝜏))    ⇒   ⊢ (𝜑 → ((𝜓 → 𝜃) ↔ (𝜒 → 𝜏)))
Theorem	bibi12d 349	Deduction joining two equivalences to form equivalence of biconditionals. (Contributed by NM, 26-May-1993.)
⊢ (𝜑 → (𝜓 ↔ 𝜒))    &   ⊢ (𝜑 → (𝜃 ↔ 𝜏))    ⇒   ⊢ (𝜑 → ((𝜓 ↔ 𝜃) ↔ (𝜒 ↔ 𝜏)))
Theorem	imbi12 350	Closed form of imbi12i 354. Was automatically derived from its "Virtual Deduction" version and the Metamath program "MM-PA> MINIMIZE_WITH *" command. (Contributed by Alan Sare, 18-Mar-2012.)
⊢ ((𝜑 ↔ 𝜓) → ((𝜒 ↔ 𝜃) → ((𝜑 → 𝜒) ↔ (𝜓 → 𝜃))))
Theorem	imbi1 351	Theorem *4.84 of [WhiteheadRussell] p. 122. (Contributed by NM, 3-Jan-2005.)
⊢ ((𝜑 ↔ 𝜓) → ((𝜑 → 𝜒) ↔ (𝜓 → 𝜒)))
Theorem	imbi2 352	Theorem *4.85 of [WhiteheadRussell] p. 122. (Contributed by NM, 3-Jan-2005.) (Proof shortened by Wolf Lammen, 19-May-2013.)
⊢ ((𝜑 ↔ 𝜓) → ((𝜒 → 𝜑) ↔ (𝜒 → 𝜓)))
Theorem	imbi1i 353	Introduce a consequent to both sides of a logical equivalence. (Contributed by NM, 3-Jan-1993.) (Proof shortened by Wolf Lammen, 17-Sep-2013.)
⊢ (𝜑 ↔ 𝜓)    ⇒   ⊢ ((𝜑 → 𝜒) ↔ (𝜓 → 𝜒))
Theorem	imbi12i 354	Join two logical equivalences to form equivalence of implications. (Contributed by NM, 1-Aug-1993.)
⊢ (𝜑 ↔ 𝜓)    &   ⊢ (𝜒 ↔ 𝜃)    ⇒   ⊢ ((𝜑 → 𝜒) ↔ (𝜓 → 𝜃))
Theorem	bibi1 355	Theorem *4.86 of [WhiteheadRussell] p. 122. (Contributed by NM, 3-Jan-2005.)
⊢ ((𝜑 ↔ 𝜓) → ((𝜑 ↔ 𝜒) ↔ (𝜓 ↔ 𝜒)))
Theorem	bitr3 356	Closed nested implication form of bitr3i 280. Derived automatically from bitr3VD 42142. (Contributed by Alan Sare, 31-Dec-2011.)
⊢ ((𝜑 ↔ 𝜓) → ((𝜑 ↔ 𝜒) → (𝜓 ↔ 𝜒)))
Theorem	con2bi 357	Contraposition. Theorem *4.12 of [WhiteheadRussell] p. 117. (Contributed by NM, 15-Apr-1995.) (Proof shortened by Wolf Lammen, 3-Jan-2013.)
⊢ ((𝜑 ↔ ¬ 𝜓) ↔ (𝜓 ↔ ¬ 𝜑))
Theorem	con2bid 358	A contraposition deduction. (Contributed by NM, 15-Apr-1995.)
⊢ (𝜑 → (𝜓 ↔ ¬ 𝜒))    ⇒   ⊢ (𝜑 → (𝜒 ↔ ¬ 𝜓))
Theorem	con1bid 359	A contraposition deduction. (Contributed by NM, 9-Oct-1999.)
⊢ (𝜑 → (¬ 𝜓 ↔ 𝜒))    ⇒   ⊢ (𝜑 → (¬ 𝜒 ↔ 𝜓))
Theorem	con1bii 360	A contraposition inference. (Contributed by NM, 12-Mar-1993.) (Proof shortened by Wolf Lammen, 13-Oct-2012.)
⊢ (¬ 𝜑 ↔ 𝜓)    ⇒   ⊢ (¬ 𝜓 ↔ 𝜑)
Theorem	con2bii 361	A contraposition inference. (Contributed by NM, 12-Mar-1993.)
⊢ (𝜑 ↔ ¬ 𝜓)    ⇒   ⊢ (𝜓 ↔ ¬ 𝜑)
Theorem	con1b 362	Contraposition. Bidirectional version of con1 148. (Contributed by NM, 3-Jan-1993.)
⊢ ((¬ 𝜑 → 𝜓) ↔ (¬ 𝜓 → 𝜑))
Theorem	con2b 363	Contraposition. Bidirectional version of con2 137. (Contributed by NM, 12-Mar-1993.)
⊢ ((𝜑 → ¬ 𝜓) ↔ (𝜓 → ¬ 𝜑))
Theorem	biimt 364	A wff is equivalent to itself with true antecedent. (Contributed by NM, 28-Jan-1996.)
⊢ (𝜑 → (𝜓 ↔ (𝜑 → 𝜓)))
Theorem	pm5.5 365	Theorem *5.5 of [WhiteheadRussell] p. 125. (Contributed by NM, 3-Jan-2005.)
⊢ (𝜑 → ((𝜑 → 𝜓) ↔ 𝜓))
Theorem	a1bi 366	Inference introducing a theorem as an antecedent. (Contributed by NM, 5-Aug-1993.) (Proof shortened by Wolf Lammen, 11-Nov-2012.)
⊢ 𝜑    ⇒   ⊢ (𝜓 ↔ (𝜑 → 𝜓))
Theorem	mt2bi 367	A false consequent falsifies an antecedent. (Contributed by NM, 19-Aug-1993.) (Proof shortened by Wolf Lammen, 12-Nov-2012.)
⊢ 𝜑    ⇒   ⊢ (¬ 𝜓 ↔ (𝜓 → ¬ 𝜑))
Theorem	mtt 368	Modus-tollens-like theorem. (Contributed by NM, 7-Apr-2001.) (Proof shortened by Wolf Lammen, 12-Nov-2012.)
⊢ (¬ 𝜑 → (¬ 𝜓 ↔ (𝜓 → 𝜑)))
Theorem	imnot 369	If a proposition is false, then implying it is equivalent to being false. One of four theorems that can be used to simplify an implication (𝜑 → 𝜓), the other ones being ax-1 6 (true consequent), pm2.21 123 (false antecedent), pm5.5 365 (true antecedent). (Contributed by Mario Carneiro, 26-Apr-2019.) (Proof shortened by Wolf Lammen, 26-May-2019.)
⊢ (¬ 𝜓 → ((𝜑 → 𝜓) ↔ ¬ 𝜑))
Theorem	pm5.501 370	Theorem *5.501 of [WhiteheadRussell] p. 125. (Contributed by NM, 3-Jan-2005.)
⊢ (𝜑 → (𝜓 ↔ (𝜑 ↔ 𝜓)))
Theorem	ibib 371	Implication in terms of implication and biconditional. (Contributed by NM, 31-Mar-1994.) (Proof shortened by Wolf Lammen, 24-Jan-2013.)
⊢ ((𝜑 → 𝜓) ↔ (𝜑 → (𝜑 ↔ 𝜓)))
Theorem	ibibr 372	Implication in terms of implication and biconditional. (Contributed by NM, 29-Apr-2005.) (Proof shortened by Wolf Lammen, 21-Dec-2013.)
⊢ ((𝜑 → 𝜓) ↔ (𝜑 → (𝜓 ↔ 𝜑)))
Theorem	tbt 373	A wff is equivalent to its equivalence with a truth. (Contributed by NM, 18-Aug-1993.) (Proof shortened by Andrew Salmon, 13-May-2011.)
⊢ 𝜑    ⇒   ⊢ (𝜓 ↔ (𝜓 ↔ 𝜑))
Theorem	nbn2 374	The negation of a wff is equivalent to the wff's equivalence to falsehood. (Contributed by Juha Arpiainen, 19-Jan-2006.) (Proof shortened by Wolf Lammen, 28-Jan-2013.)
⊢ (¬ 𝜑 → (¬ 𝜓 ↔ (𝜑 ↔ 𝜓)))
Theorem	bibif 375	Transfer negation via an equivalence. (Contributed by NM, 3-Oct-2007.) (Proof shortened by Wolf Lammen, 28-Jan-2013.)
⊢ (¬ 𝜓 → ((𝜑 ↔ 𝜓) ↔ ¬ 𝜑))
Theorem	nbn 376	The negation of a wff is equivalent to the wff's equivalence to falsehood. (Contributed by NM, 21-Jun-1993.) (Proof shortened by Wolf Lammen, 3-Oct-2013.)
⊢ ¬ 𝜑    ⇒   ⊢ (¬ 𝜓 ↔ (𝜓 ↔ 𝜑))
Theorem	nbn3 377	Transfer falsehood via equivalence. (Contributed by NM, 11-Sep-2006.)
⊢ 𝜑    ⇒   ⊢ (¬ 𝜓 ↔ (𝜓 ↔ ¬ 𝜑))
Theorem	pm5.21im 378	Two propositions are equivalent if they are both false. Closed form of 2false 379. Equivalent to a biimpr 223-like version of the xor-connective. (Contributed by Wolf Lammen, 13-May-2013.)
⊢ (¬ 𝜑 → (¬ 𝜓 → (𝜑 ↔ 𝜓)))
Theorem	2false 379	Two falsehoods are equivalent. (Contributed by NM, 4-Apr-2005.) (Proof shortened by Wolf Lammen, 19-May-2013.)
⊢ ¬ 𝜑    &   ⊢ ¬ 𝜓    ⇒   ⊢ (𝜑 ↔ 𝜓)
Theorem	2falsed 380	Two falsehoods are equivalent (deduction form). (Contributed by NM, 11-Oct-2013.) (Proof shortened by Wolf Lammen, 11-Apr-2024.)
⊢ (𝜑 → ¬ 𝜓)    &   ⊢ (𝜑 → ¬ 𝜒)    ⇒   ⊢ (𝜑 → (𝜓 ↔ 𝜒))
Theorem	2falsedOLD 381	Obsolete version of 2falsed 380 as of 11-Apr-2024. (Contributed by NM, 11-Oct-2013.) (Proof modification is discouraged.) (New usage is discouraged.)
⊢ (𝜑 → ¬ 𝜓)    &   ⊢ (𝜑 → ¬ 𝜒)    ⇒   ⊢ (𝜑 → (𝜓 ↔ 𝜒))
Theorem	pm5.21ni 382	Two propositions implying a false one are equivalent. (Contributed by NM, 16-Feb-1996.) (Proof shortened by Wolf Lammen, 19-May-2013.)
⊢ (𝜑 → 𝜓)    &   ⊢ (𝜒 → 𝜓)    ⇒   ⊢ (¬ 𝜓 → (𝜑 ↔ 𝜒))
Theorem	pm5.21nii 383	Eliminate an antecedent implied by each side of a biconditional. (Contributed by NM, 21-May-1999.)
⊢ (𝜑 → 𝜓)    &   ⊢ (𝜒 → 𝜓)    &   ⊢ (𝜓 → (𝜑 ↔ 𝜒))    ⇒   ⊢ (𝜑 ↔ 𝜒)
Theorem	pm5.21ndd 384	Eliminate an antecedent implied by each side of a biconditional, deduction version. (Contributed by Paul Chapman, 21-Nov-2012.) (Proof shortened by Wolf Lammen, 6-Oct-2013.)
⊢ (𝜑 → (𝜒 → 𝜓))    &   ⊢ (𝜑 → (𝜃 → 𝜓))    &   ⊢ (𝜑 → (𝜓 → (𝜒 ↔ 𝜃)))    ⇒   ⊢ (𝜑 → (𝜒 ↔ 𝜃))
Theorem	bija 385	Combine antecedents into a single biconditional. This inference, reminiscent of ja 189, is reversible: The hypotheses can be deduced from the conclusion alone (see pm5.1im 266 and pm5.21im 378). (Contributed by Wolf Lammen, 13-May-2013.)
⊢ (𝜑 → (𝜓 → 𝜒))    &   ⊢ (¬ 𝜑 → (¬ 𝜓 → 𝜒))    ⇒   ⊢ ((𝜑 ↔ 𝜓) → 𝜒)
Theorem	pm5.18 386	Theorem *5.18 of [WhiteheadRussell] p. 124. This theorem says that logical equivalence is the same as negated "exclusive or". (Contributed by NM, 28-Jun-2002.) (Proof shortened by Andrew Salmon, 20-Jun-2011.) (Proof shortened by Wolf Lammen, 15-Oct-2013.)
⊢ ((𝜑 ↔ 𝜓) ↔ ¬ (𝜑 ↔ ¬ 𝜓))
Theorem	xor3 387	Two ways to express "exclusive or". (Contributed by NM, 1-Jan-2006.)
⊢ (¬ (𝜑 ↔ 𝜓) ↔ (𝜑 ↔ ¬ 𝜓))
Theorem	nbbn 388	Move negation outside of biconditional. Compare Theorem *5.18 of [WhiteheadRussell] p. 124. (Contributed by NM, 27-Jun-2002.) (Proof shortened by Wolf Lammen, 20-Sep-2013.)
⊢ ((¬ 𝜑 ↔ 𝜓) ↔ ¬ (𝜑 ↔ 𝜓))
Theorem	biass 389	Associative law for the biconditional. An axiom of system DS in Vladimir Lifschitz, "On calculational proofs", Annals of Pure and Applied Logic, 113:207-224, 2002, http://www.cs.utexas.edu/users/ai-lab/pub-view.php?PubID=26805. Interestingly, this law was not included in Principia Mathematica but was apparently first noted by Jan Lukasiewicz circa 1923. (Contributed by NM, 8-Jan-2005.) (Proof shortened by Juha Arpiainen, 19-Jan-2006.) (Proof shortened by Wolf Lammen, 21-Sep-2013.)
⊢ (((𝜑 ↔ 𝜓) ↔ 𝜒) ↔ (𝜑 ↔ (𝜓 ↔ 𝜒)))
Theorem	biluk 390	Lukasiewicz's shortest axiom for equivalential calculus. Storrs McCall, ed., Polish Logic 1920-1939 (Oxford, 1967), p. 96. (Contributed by NM, 10-Jan-2005.)
⊢ ((𝜑 ↔ 𝜓) ↔ ((𝜒 ↔ 𝜓) ↔ (𝜑 ↔ 𝜒)))
Theorem	pm5.19 391	Theorem *5.19 of [WhiteheadRussell] p. 124. (Contributed by NM, 3-Jan-2005.)
⊢ ¬ (𝜑 ↔ ¬ 𝜑)
Theorem	bi2.04 392	Logical equivalence of commuted antecedents. Part of Theorem *4.87 of [WhiteheadRussell] p. 122. (Contributed by NM, 11-May-1993.)
⊢ ((𝜑 → (𝜓 → 𝜒)) ↔ (𝜓 → (𝜑 → 𝜒)))
Theorem	pm5.4 393	Antecedent absorption implication. Theorem *5.4 of [WhiteheadRussell] p. 125. (Contributed by NM, 5-Aug-1993.)
⊢ ((𝜑 → (𝜑 → 𝜓)) ↔ (𝜑 → 𝜓))
Theorem	imdi 394	Distributive law for implication. Compare Theorem *5.41 of [WhiteheadRussell] p. 125. (Contributed by NM, 5-Aug-1993.)
⊢ ((𝜑 → (𝜓 → 𝜒)) ↔ ((𝜑 → 𝜓) → (𝜑 → 𝜒)))
Theorem	pm5.41 395	Theorem *5.41 of [WhiteheadRussell] p. 125. (Contributed by NM, 3-Jan-2005.) (Proof shortened by Wolf Lammen, 12-Oct-2012.)
⊢ (((𝜑 → 𝜓) → (𝜑 → 𝜒)) ↔ (𝜑 → (𝜓 → 𝜒)))
Theorem	pm4.8 396	Theorem *4.8 of [WhiteheadRussell] p. 122. (Contributed by NM, 3-Jan-2005.)
⊢ ((𝜑 → ¬ 𝜑) ↔ ¬ 𝜑)
Theorem	pm4.81 397	A formula is equivalent to its negation implying it. Theorem *4.81 of [WhiteheadRussell] p. 122. Note that the second step, using pm2.24 124, could also use ax-1 6. (Contributed by NM, 3-Jan-2005.)
⊢ ((¬ 𝜑 → 𝜑) ↔ 𝜑)
Theorem	imim21b 398	Simplify an implication between two implications when the antecedent of the first is a consequence of the antecedent of the second. The reverse form is useful in producing the successor step in induction proofs. (Contributed by Paul Chapman, 22-Jun-2011.) (Proof shortened by Wolf Lammen, 14-Sep-2013.)
⊢ ((𝜓 → 𝜑) → (((𝜑 → 𝜒) → (𝜓 → 𝜃)) ↔ (𝜓 → (𝜒 → 𝜃))))
1.2.6  Logical conjunction This section defines conjunction of two formulas, denoted by infix "∧ " and read "and". It is defined in terms of implication and negation, which is possible in classical logic (but not in intuitionistic logic: see iset.mm). After the definition, we briefly introduce conversion of simple expressions to and from conjunction. Two simple operations called importation (imp 410) and exportation (ex 416) follow. In the propositions-as-types interpretation, they correspond to uncurrying and currying respectively. They are foundational for this section. Most of the theorems proved here trace back to them, mostly indirectly, in a layered fashion, where more complex expressions are built from simpler ones. Here are some of these successive layers: importation and exportation, commutativity and associativity laws, adding antecedents and simplifying, conjunction of consequents, syllogisms, etc. As indicated in the "note on definitions" in the section comment for logical equivalence, some theorems containing only implication, negation and conjunction are placed in the section after disjunction since theirs proofs use disjunction (although this is not required since definitions are conservative, see said section comment).
Syntax	wa 399	Extend wff definition to include conjunction ("and").
wff (𝜑 ∧ 𝜓)
Definition	df-an 400	Define conjunction (logical "and"). Definition of [Margaris] p. 49. When both the left and right operand are true, the result is true; when either is false, the result is false. For example, it is true that (2 = 2 ∧ 3 = 3). After we define the constant true ⊤ (df-tru 1546) and the constant false ⊥ (df-fal 1556), we will be able to prove these truth table values: ((⊤ ∧ ⊤) ↔ ⊤) (truantru 1576), ((⊤ ∧ ⊥) ↔ ⊥) (truanfal 1577), ((⊥ ∧ ⊤) ↔ ⊥) (falantru 1578), and ((⊥ ∧ ⊥) ↔ ⊥) (falanfal 1579). This is our first use of the biconditional connective in a definition; we use the biconditional connective in place of the traditional "<=def=>", which means the same thing, except that we can manipulate the biconditional connective directly in proofs rather than having to rely on an informal definition substitution rule. Note that if we mechanically substitute ¬ (𝜑 → ¬ 𝜓) for (𝜑 ∧ 𝜓), we end up with an instance of previously proved theorem biid 264. This is the justification for the definition, along with the fact that it introduces a new symbol ∧. Contrast with ∨ (df-or 848), → (wi 4), ⊼ (df-nan 1488), and ⊻ (df-xor 1508). (Contributed by NM, 5-Jan-1993.)
⊢ ((𝜑 ∧ 𝜓) ↔ ¬ (𝜑 → ¬ 𝜓))