P. 17 of Theorem List - Intuitionistic Logic Explorer

Theorem List for Intuitionistic Logic Explorer - 1601-1700   *Has distinct variable group(s)

Type	Label	Description
Statement
Theorem	nexd 1601	Deduction for generalization rule for negated wff. (Contributed by NM, 2-Jan-2002.)
|- ( ph -> A. x ph )   &    |- ( ph -> -. ps )   =>    |- ( ph -> -. E. x ps )
Theorem	exbidh 1602	Formula-building rule for existential quantifier (deduction form). (Contributed by NM, 5-Aug-1993.)
|- ( ph -> A. x ph )   &    |- ( ph -> ( ps <-> ch ) )   =>    |- ( ph -> ( E. x ps <-> E. x ch ) )
Theorem	albid 1603	Formula-building rule for universal quantifier (deduction form). (Contributed by Mario Carneiro, 24-Sep-2016.)
|- F/ x ph   &    |- ( ph -> ( ps <-> ch ) )   =>    |- ( ph -> ( A. x ps <-> A. x ch ) )
Theorem	exbid 1604	Formula-building rule for existential quantifier (deduction form). (Contributed by Mario Carneiro, 24-Sep-2016.)
|- F/ x ph   &    |- ( ph -> ( ps <-> ch ) )   =>    |- ( ph -> ( E. x ps <-> E. x ch ) )
Theorem	exsimpl 1605	Simplification of an existentially quantified conjunction. (Contributed by Rodolfo Medina, 25-Sep-2010.) (Proof shortened by Andrew Salmon, 29-Jun-2011.)
|- ( E. x ( ph /\ ps ) -> E. x ph )
Theorem	exsimpr 1606	Simplification of an existentially quantified conjunction. (Contributed by Rodolfo Medina, 25-Sep-2010.) (Proof shortened by Andrew Salmon, 29-Jun-2011.)
|- ( E. x ( ph /\ ps ) -> E. x ps )
Theorem	alexdc 1607	Theorem 19.6 of [Margaris] p. 89, given a decidability condition. The forward direction holds for all propositions, as seen at alexim 1633. (Contributed by Jim Kingdon, 2-Jun-2018.)
|- ( A. xDECID ph -> ( A. x ph <-> -. E. x -. ph ) )
Theorem	19.29 1608	Theorem 19.29 of [Margaris] p. 90. (Contributed by NM, 5-Aug-1993.) (Proof shortened by Andrew Salmon, 13-May-2011.)
|- ( ( A. x ph /\ E. x ps ) -> E. x ( ph /\ ps ) )
Theorem	19.29r 1609	Variation of Theorem 19.29 of [Margaris] p. 90. (Contributed by NM, 18-Aug-1993.)
|- ( ( E. x ph /\ A. x ps ) -> E. x ( ph /\ ps ) )
Theorem	19.29r2 1610	Variation of Theorem 19.29 of [Margaris] p. 90 with double quantification. (Contributed by NM, 3-Feb-2005.)
|- ( ( E. x E. y ph /\ A. x A. y ps ) -> E. x E. y ( ph /\ ps ) )
Theorem	19.29x 1611	Variation of Theorem 19.29 of [Margaris] p. 90 with mixed quantification. (Contributed by NM, 11-Feb-2005.)
|- ( ( E. x A. y ph /\ A. x E. y ps ) -> E. x E. y ( ph /\ ps ) )
Theorem	19.35-1 1612	Forward direction of Theorem 19.35 of [Margaris] p. 90. The converse holds for classical logic but not (for all propositions) in intuitionistic logic. (Contributed by Mario Carneiro, 2-Feb-2015.)
|- ( E. x ( ph -> ps ) -> ( A. x ph -> E. x ps ) )
Theorem	19.35i 1613	Inference from Theorem 19.35 of [Margaris] p. 90. (Contributed by NM, 5-Aug-1993.) (Revised by NM, 2-Feb-2015.)
|- E. x ( ph -> ps )   =>    |- ( A. x ph -> E. x ps )
Theorem	19.25 1614	Theorem 19.25 of [Margaris] p. 90. (Contributed by NM, 5-Aug-1993.) (Revised by NM, 2-Feb-2015.)
|- ( A. y E. x ( ph -> ps ) -> ( E. y A. x ph -> E. y E. x ps ) )
Theorem	19.30dc 1615	Theorem 19.30 of [Margaris] p. 90, with an additional decidability condition. (Contributed by Jim Kingdon, 21-Jul-2018.)
|- (DECID E. x ps -> ( A. x ( ph \/ ps ) -> ( A. x ph \/ E. x ps ) ) )
Theorem	19.43 1616	Theorem 19.43 of [Margaris] p. 90. (Contributed by NM, 5-Aug-1993.) (Proof shortened by Mario Carneiro, 2-Feb-2015.)
|- ( E. x ( ph \/ ps ) <-> ( E. x ph \/ E. x ps ) )
Theorem	19.33b2 1617	The antecedent provides a condition implying the converse of 19.33 1472. Compare Theorem 19.33 of [Margaris] p. 90. This variation of 19.33bdc 1618 is intuitionistically valid without a decidability condition. (Contributed by Mario Carneiro, 2-Feb-2015.)
|- ( ( -. E. x ph \/ -. E. x ps ) -> ( A. x ( ph \/ ps ) <-> ( A. x ph \/ A. x ps ) ) )
Theorem	19.33bdc 1618	Converse of 19.33 1472 given -. ( E. x ph /\ E. x ps ) and a decidability condition. Compare Theorem 19.33 of [Margaris] p. 90. For a version which does not require a decidability condition, see 19.33b2 1617 (Contributed by Jim Kingdon, 23-Apr-2018.)
|- (DECID E. x ph -> ( -. ( E. x ph /\ E. x ps ) -> ( A. x ( ph \/ ps ) <-> ( A. x ph \/ A. x ps ) ) ) )
Theorem	19.40 1619	Theorem 19.40 of [Margaris] p. 90. (Contributed by NM, 5-Aug-1993.)
|- ( E. x ( ph /\ ps ) -> ( E. x ph /\ E. x ps ) )
Theorem	19.40-2 1620	Theorem *11.42 in [WhiteheadRussell] p. 163. Theorem 19.40 of [Margaris] p. 90 with 2 quantifiers. (Contributed by Andrew Salmon, 24-May-2011.)
|- ( E. x E. y ( ph /\ ps ) -> ( E. x E. y ph /\ E. x E. y ps ) )
Theorem	exintrbi 1621	Add/remove a conjunct in the scope of an existential quantifier. (Contributed by Raph Levien, 3-Jul-2006.)
|- ( A. x ( ph -> ps ) -> ( E. x ph <-> E. x ( ph /\ ps ) ) )
Theorem	exintr 1622	Introduce a conjunct in the scope of an existential quantifier. (Contributed by NM, 11-Aug-1993.)
|- ( A. x ( ph -> ps ) -> ( E. x ph -> E. x ( ph /\ ps ) ) )
Theorem	alsyl 1623	Theorem *10.3 in [WhiteheadRussell] p. 150. (Contributed by Andrew Salmon, 8-Jun-2011.)
|- ( ( A. x ( ph -> ps ) /\ A. x ( ps -> ch ) ) -> A. x ( ph -> ch ) )
Theorem	hbex 1624	If x is not free in ph, it is not free in E. y ph. (Contributed by NM, 5-Aug-1993.) (Revised by NM, 2-Feb-2015.)
|- ( ph -> A. x ph )   =>    |- ( E. y ph -> A. x E. y ph )
Theorem	nfex 1625	If x is not free in ph, it is not free in E. y ph. (Contributed by Mario Carneiro, 11-Aug-2016.) (Proof shortened by Wolf Lammen, 30-Dec-2017.)
|- F/ x ph   =>    |- F/ x E. y ph
Theorem	19.2 1626	Theorem 19.2 of [Margaris] p. 89, generalized to use two setvar variables. (Contributed by O'Cat, 31-Mar-2008.)
|- ( A. x ph -> E. y ph )
Theorem	i19.24 1627	Theorem 19.24 of [Margaris] p. 90, with an additional hypothesis. The hypothesis is the converse of 19.35-1 1612, and is a theorem of classical logic, but in intuitionistic logic it will only be provable for some propositions. (Contributed by Jim Kingdon, 22-Jul-2018.)
|- ( ( A. x ph -> E. x ps ) -> E. x ( ph -> ps ) )   =>    |- ( ( A. x ph -> A. x ps ) -> E. x ( ph -> ps ) )
Theorem	i19.39 1628	Theorem 19.39 of [Margaris] p. 90, with an additional hypothesis. The hypothesis is the converse of 19.35-1 1612, and is a theorem of classical logic, but in intuitionistic logic it will only be provable for some propositions. (Contributed by Jim Kingdon, 22-Jul-2018.)
|- ( ( A. x ph -> E. x ps ) -> E. x ( ph -> ps ) )   =>    |- ( ( E. x ph -> E. x ps ) -> E. x ( ph -> ps ) )
Theorem	19.9ht 1629	A closed version of one direction of 19.9 1632. (Contributed by NM, 5-Aug-1993.)
|- ( A. x ( ph -> A. x ph ) -> ( E. x ph -> ph ) )
Theorem	19.9t 1630	A closed version of 19.9 1632. (Contributed by NM, 5-Aug-1993.) (Revised by Mario Carneiro, 24-Sep-2016.) (Proof shortended by Wolf Lammen, 30-Dec-2017.)
|- ( F/ x ph -> ( E. x ph <-> ph ) )
Theorem	19.9h 1631	A wff may be existentially quantified with a variable not free in it. Theorem 19.9 of [Margaris] p. 89. (Contributed by FL, 24-Mar-2007.)
|- ( ph -> A. x ph )   =>    |- ( E. x ph <-> ph )
Theorem	19.9 1632	A wff may be existentially quantified with a variable not free in it. Theorem 19.9 of [Margaris] p. 89. (Contributed by FL, 24-Mar-2007.) (Revised by Mario Carneiro, 24-Sep-2016.) (Proof shortened by Wolf Lammen, 30-Dec-2017.)
|- F/ x ph   =>    |- ( E. x ph <-> ph )
Theorem	alexim 1633	One direction of theorem 19.6 of [Margaris] p. 89. The converse holds given a decidability condition, as seen at alexdc 1607. (Contributed by Jim Kingdon, 2-Jul-2018.)
|- ( A. x ph -> -. E. x -. ph )
Theorem	exnalim 1634	One direction of Theorem 19.14 of [Margaris] p. 90. In classical logic the converse also holds. (Contributed by Jim Kingdon, 15-Jul-2018.)
|- ( E. x -. ph -> -. A. x ph )
Theorem	exanaliim 1635	A transformation of quantifiers and logical connectives. In classical logic the converse also holds. (Contributed by Jim Kingdon, 15-Jul-2018.)
|- ( E. x ( ph /\ -. ps ) -> -. A. x ( ph -> ps ) )
Theorem	alexnim 1636	A relationship between two quantifiers and negation. (Contributed by Jim Kingdon, 27-Aug-2018.)
|- ( A. x E. y -. ph -> -. E. x A. y ph )
Theorem	nnal 1637	The double negation of a universal quantification implies the universal quantification of the double negation. (Contributed by BJ, 24-Nov-2023.)
|- ( -. -. A. x ph -> A. x -. -. ph )
Theorem	ax6blem 1638	If x is not free in ph, it is not free in -. ph. This theorem doesn't use ax6b 1639 compared to hbnt 1641. (Contributed by GD, 27-Jan-2018.)
|- ( ph -> A. x ph )   =>    |- ( -. ph -> A. x -. ph )
Theorem	ax6b 1639	Quantified Negation. Axiom C5-2 of [Monk2] p. 113. (Contributed by GD, 27-Jan-2018.)
|- ( -. A. x ph -> A. x -. A. x ph )
Theorem	hbn1 1640	x is not free in -. A. x ph. (Contributed by NM, 5-Aug-1993.) (Proof shortened by Wolf Lammen, 18-Aug-2014.)
|- ( -. A. x ph -> A. x -. A. x ph )
Theorem	hbnt 1641	Closed theorem version of bound-variable hypothesis builder hbn 1642. (Contributed by NM, 5-Aug-1993.) (Revised by NM, 2-Feb-2015.)
|- ( A. x ( ph -> A. x ph ) -> ( -. ph -> A. x -. ph ) )
Theorem	hbn 1642	If x is not free in ph, it is not free in -. ph. (Contributed by NM, 5-Aug-1993.)
|- ( ph -> A. x ph )   =>    |- ( -. ph -> A. x -. ph )
Theorem	hbnd 1643	Deduction form of bound-variable hypothesis builder hbn 1642. (Contributed by NM, 3-Jan-2002.)
|- ( ph -> A. x ph )   &    |- ( ph -> ( ps -> A. x ps ) )   =>    |- ( ph -> ( -. ps -> A. x -. ps ) )
Theorem	nfnt 1644	If x is not free in ph, then it is not free in -. ph. (Contributed by Mario Carneiro, 24-Sep-2016.) (Proof shortened by Wolf Lammen, 28-Dec-2017.) (Revised by BJ, 24-Jul-2019.)
|- ( F/ x ph -> F/ x -. ph )
Theorem	nfnd 1645	Deduction associated with nfnt 1644. (Contributed by Mario Carneiro, 24-Sep-2016.)
|- ( ph -> F/ x ps )   =>    |- ( ph -> F/ x -. ps )
Theorem	nfn 1646	Inference associated with nfnt 1644. (Contributed by Mario Carneiro, 11-Aug-2016.)
|- F/ x ph   =>    |- F/ x -. ph
Theorem	nfdc 1647	If x is not free in ph, it is not free in DECID ph. (Contributed by Jim Kingdon, 11-Mar-2018.)
|- F/ x ph   =>    |- F/ xDECID ph
Theorem	modal-5 1648	The analog in our predicate calculus of axiom 5 of modal logic S5. (Contributed by NM, 5-Oct-2005.)
|- ( -. A. x -. ph -> A. x -. A. x -. ph )
Theorem	19.9d 1649	A deduction version of one direction of 19.9 1632. (Contributed by NM, 5-Aug-1993.) (Revised by Mario Carneiro, 24-Sep-2016.)
|- ( ps -> F/ x ph )   =>    |- ( ps -> ( E. x ph -> ph ) )
Theorem	19.9hd 1650	A deduction version of one direction of 19.9 1632. This is an older variation of this theorem; new proofs should use 19.9d 1649. (Contributed by NM, 5-Aug-1993.) (New usage is discouraged.)
|- ( ps -> A. x ps )   &    |- ( ps -> ( ph -> A. x ph ) )   =>    |- ( ps -> ( E. x ph -> ph ) )
Theorem	excomim 1651	One direction of Theorem 19.11 of [Margaris] p. 89. (Contributed by NM, 5-Aug-1993.)
|- ( E. x E. y ph -> E. y E. x ph )
Theorem	excom 1652	Theorem 19.11 of [Margaris] p. 89. (Contributed by NM, 5-Aug-1993.)
|- ( E. x E. y ph <-> E. y E. x ph )
Theorem	19.12 1653	Theorem 19.12 of [Margaris] p. 89. Assuming the converse is a mistake sometimes made by beginners! (Contributed by NM, 5-Aug-1993.)
|- ( E. x A. y ph -> A. y E. x ph )
Theorem	19.19 1654	Theorem 19.19 of [Margaris] p. 90. (Contributed by NM, 12-Mar-1993.)
|- F/ x ph   =>    |- ( A. x ( ph <-> ps ) -> ( ph <-> E. x ps ) )
Theorem	19.21-2 1655	Theorem 19.21 of [Margaris] p. 90 but with 2 quantifiers. (Contributed by NM, 4-Feb-2005.)
|- F/ x ph   &    |- F/ y ph   =>    |- ( A. x A. y ( ph -> ps ) <-> ( ph -> A. x A. y ps ) )
Theorem	nf2 1656	An alternate definition of df-nf 1449, which does not involve nested quantifiers on the same variable. (Contributed by Mario Carneiro, 24-Sep-2016.)
|- ( F/ x ph <-> ( E. x ph -> A. x ph ) )
Theorem	nf3 1657	An alternate definition of df-nf 1449. (Contributed by Mario Carneiro, 24-Sep-2016.)
|- ( F/ x ph <-> A. x ( E. x ph -> ph ) )
Theorem	nf4dc 1658	Variable x is effectively not free in ph iff ph is always true or always false, given a decidability condition. The reverse direction, nf4r 1659, holds for all propositions. (Contributed by Jim Kingdon, 21-Jul-2018.)
|- (DECID E. x ph -> ( F/ x ph <-> ( A. x ph \/ A. x -. ph ) ) )
Theorem	nf4r 1659	If ph is always true or always false, then variable x is effectively not free in ph. The converse holds given a decidability condition, as seen at nf4dc 1658. (Contributed by Jim Kingdon, 21-Jul-2018.)
|- ( ( A. x ph \/ A. x -. ph ) -> F/ x ph )
Theorem	19.36i 1660	Inference from Theorem 19.36 of [Margaris] p. 90. (Contributed by NM, 5-Aug-1993.) (Revised by NM, 2-Feb-2015.)
|- F/ x ps   &    |- E. x ( ph -> ps )   =>    |- ( A. x ph -> ps )
Theorem	19.36-1 1661	Closed form of 19.36i 1660. One direction of Theorem 19.36 of [Margaris] p. 90. The converse holds in classical logic, but does not hold (for all propositions) in intuitionistic logic. (Contributed by Jim Kingdon, 20-Jun-2018.)
|- F/ x ps   =>    |- ( E. x ( ph -> ps ) -> ( A. x ph -> ps ) )
Theorem	19.37-1 1662	One direction of Theorem 19.37 of [Margaris] p. 90. The converse holds in classical logic but not, in general, here. (Contributed by Jim Kingdon, 21-Jun-2018.)
|- F/ x ph   =>    |- ( E. x ( ph -> ps ) -> ( ph -> E. x ps ) )
Theorem	19.37aiv 1663*	Inference from Theorem 19.37 of [Margaris] p. 90. (Contributed by NM, 5-Aug-1993.)
|- E. x ( ph -> ps )   =>    |- ( ph -> E. x ps )
Theorem	19.38 1664	Theorem 19.38 of [Margaris] p. 90. (Contributed by NM, 5-Aug-1993.)
|- ( ( E. x ph -> A. x ps ) -> A. x ( ph -> ps ) )
Theorem	19.23t 1665	Closed form of Theorem 19.23 of [Margaris] p. 90. (Contributed by NM, 7-Nov-2005.) (Proof shortened by Wolf Lammen, 2-Jan-2018.)
|- ( F/ x ps -> ( A. x ( ph -> ps ) <-> ( E. x ph -> ps ) ) )
Theorem	19.23 1666	Theorem 19.23 of [Margaris] p. 90. (Contributed by NM, 5-Aug-1993.) (Revised by Mario Carneiro, 24-Sep-2016.)
|- F/ x ps   =>    |- ( A. x ( ph -> ps ) <-> ( E. x ph -> ps ) )
Theorem	19.32dc 1667	Theorem 19.32 of [Margaris] p. 90, where ph is decidable. (Contributed by Jim Kingdon, 4-Jun-2018.)
|- F/ x ph   =>    |- (DECID ph -> ( A. x ( ph \/ ps ) <-> ( ph \/ A. x ps ) ) )
Theorem	19.32r 1668	One direction of Theorem 19.32 of [Margaris] p. 90. The converse holds if ph is decidable, as seen at 19.32dc 1667. (Contributed by Jim Kingdon, 28-Jul-2018.)
|- F/ x ph   =>    |- ( ( ph \/ A. x ps ) -> A. x ( ph \/ ps ) )
Theorem	19.31r 1669	One direction of Theorem 19.31 of [Margaris] p. 90. The converse holds in classical logic, but not intuitionistic logic. (Contributed by Jim Kingdon, 28-Jul-2018.)
|- F/ x ps   =>    |- ( ( A. x ph \/ ps ) -> A. x ( ph \/ ps ) )
Theorem	19.44 1670	Theorem 19.44 of [Margaris] p. 90. (Contributed by NM, 12-Mar-1993.)
|- F/ x ps   =>    |- ( E. x ( ph \/ ps ) <-> ( E. x ph \/ ps ) )
Theorem	19.45 1671	Theorem 19.45 of [Margaris] p. 90. (Contributed by NM, 12-Mar-1993.)
|- F/ x ph   =>    |- ( E. x ( ph \/ ps ) <-> ( ph \/ E. x ps ) )
Theorem	19.34 1672	Theorem 19.34 of [Margaris] p. 90. (Contributed by NM, 5-Aug-1993.)
|- ( ( A. x ph \/ E. x ps ) -> E. x ( ph \/ ps ) )
Theorem	19.41h 1673	Theorem 19.41 of [Margaris] p. 90. New proofs should use 19.41 1674 instead. (Contributed by NM, 5-Aug-1993.) (Proof shortened by Andrew Salmon, 25-May-2011.) (New usage is discouraged.)
|- ( ps -> A. x ps )   =>    |- ( E. x ( ph /\ ps ) <-> ( E. x ph /\ ps ) )
Theorem	19.41 1674	Theorem 19.41 of [Margaris] p. 90. (Contributed by NM, 5-Aug-1993.) (Proof shortened by Andrew Salmon, 25-May-2011.) (Proof shortened by Wolf Lammen, 12-Jan-2018.)
|- F/ x ps   =>    |- ( E. x ( ph /\ ps ) <-> ( E. x ph /\ ps ) )
Theorem	19.42h 1675	Theorem 19.42 of [Margaris] p. 90. New proofs should use 19.42 1676 instead. (Contributed by NM, 18-Aug-1993.) (New usage is discouraged.)
|- ( ph -> A. x ph )   =>    |- ( E. x ( ph /\ ps ) <-> ( ph /\ E. x ps ) )
Theorem	19.42 1676	Theorem 19.42 of [Margaris] p. 90. (Contributed by NM, 18-Aug-1993.)
|- F/ x ph   =>    |- ( E. x ( ph /\ ps ) <-> ( ph /\ E. x ps ) )
Theorem	excom13 1677	Swap 1st and 3rd existential quantifiers. (Contributed by NM, 9-Mar-1995.)
|- ( E. x E. y E. z ph <-> E. z E. y E. x ph )
Theorem	exrot3 1678	Rotate existential quantifiers. (Contributed by NM, 17-Mar-1995.)
|- ( E. x E. y E. z ph <-> E. y E. z E. x ph )
Theorem	exrot4 1679	Rotate existential quantifiers twice. (Contributed by NM, 9-Mar-1995.)
|- ( E. x E. y E. z E. w ph <-> E. z E. w E. x E. y ph )
Theorem	nexr 1680	Inference from 19.8a 1578. (Contributed by Jeff Hankins, 26-Jul-2009.)
|- -. E. x ph   =>    |- -. ph
Theorem	exan 1681	Place a conjunct in the scope of an existential quantifier. (Contributed by NM, 18-Aug-1993.) (Proof shortened by Andrew Salmon, 25-May-2011.)
|- ( E. x ph /\ ps )   =>    |- E. x ( ph /\ ps )
Theorem	hbexd 1682	Deduction form of bound-variable hypothesis builder hbex 1624. (Contributed by NM, 2-Jan-2002.)
|- ( ph -> A. y ph )   &    |- ( ph -> ( ps -> A. x ps ) )   =>    |- ( ph -> ( E. y ps -> A. x E. y ps ) )
Theorem	eeor 1683	Rearrange existential quantifiers. (Contributed by NM, 8-Aug-1994.)
|- F/ y ph   &    |- F/ x ps   =>    |- ( E. x E. y ( ph \/ ps ) <-> ( E. x ph \/ E. y ps ) )
1.3.8  Equality theorems without distinct variables
Theorem	a9e 1684	At least one individual exists. This is not a theorem of free logic, which is sound in empty domains. For such a logic, we would add this theorem as an axiom of set theory (Axiom 0 of [Kunen] p. 10). In the system consisting of ax-5 1435 through ax-14 2139 and ax-17 1514, all axioms other than ax-9 1519 are believed to be theorems of free logic, although the system without ax-9 1519 is probably not complete in free logic. (Contributed by NM, 5-Aug-1993.) (Revised by NM, 3-Feb-2015.)
|- E. x x = y
Theorem	a9ev 1685*	At least one individual exists. Weaker version of a9e 1684. (Contributed by NM, 3-Aug-2017.)
|- E. x x = y
Theorem	ax9o 1686	An implication related to substitution. (Contributed by NM, 5-Aug-1993.) (Revised by NM, 3-Feb-2015.)
|- ( A. x ( x = y -> A. x ph ) -> ph )
Theorem	spimfv 1687*	Specialization, using implicit substitution. Version of spim 1726 with a disjoint variable condition. See spimv 1799 for another variant. (Contributed by NM, 10-Jan-1993.) (Revised by BJ, 31-May-2019.)
|- F/ x ps   &    |- ( x = y -> ( ph -> ps ) )   =>    |- ( A. x ph -> ps )
Theorem	chvarfv 1688*	Implicit substitution of y for x into a theorem. Version of chvar 1745 with a disjoint variable condition. (Contributed by Raph Levien, 9-Jul-2003.) (Revised by BJ, 31-May-2019.)
|- F/ x ps   &    |- ( x = y -> ( ph <-> ps ) )   &    |- ph   =>    |- ps
Theorem	equid 1689	Identity law for equality (reflexivity). Lemma 6 of [Tarski] p. 68. This is often an axiom of equality in textbook systems, but we don't need it as an axiom since it can be proved from our other axioms. This proof is similar to Tarski's and makes use of a dummy variable y. It also works in intuitionistic logic, unlike some other possible ways of proving this theorem. (Contributed by NM, 1-Apr-2005.)
|- x = x
Theorem	nfequid 1690	Bound-variable hypothesis builder for x = x. This theorem tells us that any variable, including x, is effectively not free in x = x, even though x is technically free according to the traditional definition of free variable. (Contributed by NM, 13-Jan-2011.) (Revised by NM, 21-Aug-2017.)
|- F/ y x = x
Theorem	stdpc6 1691	One of the two equality axioms of standard predicate calculus, called reflexivity of equality. (The other one is stdpc7 1758.) Axiom 6 of [Mendelson] p. 95. Mendelson doesn't say why he prepended the redundant quantifier, but it was probably to be compatible with free logic (which is valid in the empty domain). (Contributed by NM, 16-Feb-2005.)
|- A. x x = x
Theorem	equcomi 1692	Commutative law for equality. Lemma 7 of [Tarski] p. 69. (Contributed by NM, 5-Aug-1993.)
|- ( x = y -> y = x )
Theorem	ax6evr 1693*	A commuted form of a9ev 1685. The naming reflects how axioms were numbered in the Metamath Proof Explorer as of 2020 (a numbering which we eventually plan to adopt here too, but until this happens everywhere only some theorems will have it). (Contributed by BJ, 7-Dec-2020.)
|- E. x y = x
Theorem	equcom 1694	Commutative law for equality. (Contributed by NM, 20-Aug-1993.)
|- ( x = y <-> y = x )
Theorem	equcomd 1695	Deduction form of equcom 1694, symmetry of equality. For the versions for classes, see eqcom 2167 and eqcomd 2171. (Contributed by BJ, 6-Oct-2019.)
|- ( ph -> x = y )   =>    |- ( ph -> y = x )
Theorem	equcoms 1696	An inference commuting equality in antecedent. Used to eliminate the need for a syllogism. (Contributed by NM, 5-Aug-1993.)
|- ( x = y -> ph )   =>    |- ( y = x -> ph )
Theorem	equtr 1697	A transitive law for equality. (Contributed by NM, 23-Aug-1993.)
|- ( x = y -> ( y = z -> x = z ) )
Theorem	equtrr 1698	A transitive law for equality. Lemma L17 in [Megill] p. 446 (p. 14 of the preprint). (Contributed by NM, 23-Aug-1993.)
|- ( x = y -> ( z = x -> z = y ) )
Theorem	equtr2 1699	A transitive law for equality. (Contributed by NM, 12-Aug-1993.) (Proof shortened by Andrew Salmon, 25-May-2011.)
|- ( ( x = z /\ y = z ) -> x = y )
Theorem	equequ1 1700	An equivalence law for equality. (Contributed by NM, 5-Aug-1993.)
|- ( x = y -> ( x = z <-> y = z ) )