P. 27 of Theorem List - Metamath Proof Explorer

Theorem List for Metamath Proof Explorer - 2601-2700   *Has distinct variable group(s)

Type	Label	Description
Statement
Theorem	sbab 2601*	The right-hand side of the second equality is a way of representing proper substitution of y for x into a class variable. (Contributed by NM, 14-Sep-2003.)
|- ( x = y -> A = { z | [ y / x ] z e. A } )
2.1.3  Class form not-free predicate
Syntax	wnfc 2602	Extend wff definition to include the not-free predicate for classes.
wff F/_ x A
Theorem	nfcjust 2603*	Justification theorem for df-nfc 2604. (Contributed by Mario Carneiro, 13-Oct-2016.)
|- ( A. y F/ x y e. A <-> A. z F/ x z e. A )
Definition	df-nfc 2604*	Define the not-free predicate for classes. This is read " x is not free in A". Not-free means that the value of x cannot affect the value of A, e.g., any occurrence of x in A is effectively bound by a "for all" or something that expands to one (such as "there exists"). It is defined in terms of the not-free predicate df-nf 1622 for wffs; see that definition for more information. (Contributed by Mario Carneiro, 11-Aug-2016.)
|- ( F/_ x A <-> A. y F/ x y e. A )
Theorem	nfci 2605*	Deduce that a class A does not have x free in it. (Contributed by Mario Carneiro, 11-Aug-2016.)
|- F/ x y e. A   =>    |- F/_ x A
Theorem	nfcii 2606*	Deduce that a class A does not have x free in it. (Contributed by Mario Carneiro, 11-Aug-2016.)
|- ( y e. A -> A. x y e. A )   =>    |- F/_ x A
Theorem	nfcr 2607*	Consequence of the not-free predicate. (Contributed by Mario Carneiro, 11-Aug-2016.)
|- ( F/_ x A -> F/ x y e. A )
Theorem	nfcrii 2608*	Consequence of the not-free predicate. (Contributed by Mario Carneiro, 11-Aug-2016.)
|- F/_ x A   =>    |- ( y e. A -> A. x y e. A )
Theorem	nfcri 2609*	Consequence of the not-free predicate. (Note that unlike nfcr 2607, this does not require y and A to be disjoint.) (Contributed by Mario Carneiro, 11-Aug-2016.)
|- F/_ x A   =>    |- F/ x y e. A
Theorem	nfcd 2610*	Deduce that a class A does not have x free in it. (Contributed by Mario Carneiro, 11-Aug-2016.)
|- F/ y ph   &    |- ( ph -> F/ x y e. A )   =>    |- ( ph -> F/_ x A )
Theorem	nfceqdf 2611	An equality theorem for effectively not free. (Contributed by Mario Carneiro, 14-Oct-2016.)
|- F/ x ph   &    |- ( ph -> A = B )   =>    |- ( ph -> ( F/_ x A <-> F/_ x B ) )
Theorem	nfceqi 2612	Equality theorem for class not-free. (Contributed by Mario Carneiro, 11-Aug-2016.) (Proof shortened by Wolf Lammen, 16-Nov-2019.)
|- A = B   =>    |- ( F/_ x A <-> F/_ x B )
Theorem	nfceqiOLD 2613	Obsolete proof of nfceqi 2612 as of 16-Nov-2019. (Contributed by Mario Carneiro, 11-Aug-2016.) (Proof modification is discouraged.) (New usage is discouraged.)
|- A = B   =>    |- ( F/_ x A <-> F/_ x B )
Theorem	nfcxfr 2614	A utility lemma to transfer a bound-variable hypothesis builder into a definition. (Contributed by Mario Carneiro, 11-Aug-2016.)
|- A = B   &    |- F/_ x B   =>    |- F/_ x A
Theorem	nfcxfrd 2615	A utility lemma to transfer a bound-variable hypothesis builder into a definition. (Contributed by Mario Carneiro, 11-Aug-2016.)
|- A = B   &    |- ( ph -> F/_ x B )   =>    |- ( ph -> F/_ x A )
Theorem	nfcv 2616*	If x is disjoint from A, then x is not free in A. (Contributed by Mario Carneiro, 11-Aug-2016.)
|- F/_ x A
Theorem	nfcvd 2617*	If x is disjoint from A, then x is not free in A. (Contributed by Mario Carneiro, 7-Oct-2016.)
|- ( ph -> F/_ x A )
Theorem	nfab1 2618	Bound-variable hypothesis builder for a class abstraction. (Contributed by Mario Carneiro, 11-Aug-2016.)
|- F/_ x { x | ph }
Theorem	nfnfc1 2619	x is bound in F/_ x A. (Contributed by Mario Carneiro, 11-Aug-2016.)
|- F/ x F/_ x A
Theorem	nfab 2620	Bound-variable hypothesis builder for a class abstraction. (Contributed by Mario Carneiro, 11-Aug-2016.)
|- F/ x ph   =>    |- F/_ x { y | ph }
Theorem	nfaba1 2621	Bound-variable hypothesis builder for a class abstraction. (Contributed by Mario Carneiro, 14-Oct-2016.)
|- F/_ x { y | A. x ph }
Theorem	nfcrd 2622*	Consequence of the not-free predicate. (Contributed by Mario Carneiro, 11-Aug-2016.)
|- ( ph -> F/_ x A )   =>    |- ( ph -> F/ x y e. A )
Theorem	nfeqd 2623	Hypothesis builder for equality. (Contributed by Mario Carneiro, 7-Oct-2016.)
|- ( ph -> F/_ x A )   &    |- ( ph -> F/_ x B )   =>    |- ( ph -> F/ x A = B )
Theorem	nfeld 2624	Hypothesis builder for elementhood. (Contributed by Mario Carneiro, 7-Oct-2016.)
|- ( ph -> F/_ x A )   &    |- ( ph -> F/_ x B )   =>    |- ( ph -> F/ x A e. B )
Theorem	nfnfc 2625	Hypothesis builder for F/_ y A. (Contributed by Mario Carneiro, 11-Aug-2016.) Remove dependency on ax-13 2004. (Revised by Wolf Lammen, 10-Dec-2019.)
|- F/_ x A   =>    |- F/ x F/_ y A
Theorem	nfnfcALT 2626	Alternative, shorter proof of nfnfc 2625, based on more axioms. (Contributed by Mario Carneiro, 11-Aug-2016.) (New usage is discouraged.) (Proof modification is discouraged.)
|- F/_ x A   =>    |- F/ x F/_ y A
Theorem	nfeq 2627	Hypothesis builder for equality. (Contributed by NM, 21-Jun-1993.) (Revised by Mario Carneiro, 11-Aug-2016.) (Proof shortened by Wolf Lammen, 16-Nov-2019.)
|- F/_ x A   &    |- F/_ x B   =>    |- F/ x A = B
Theorem	nfeqOLD 2628	Obsolete proof of nfeq 2627 as of 16-Nov-2019. (Contributed by NM, 21-Jun-1993.) (Revised by Mario Carneiro, 11-Aug-2016.) (New usage is discouraged.) (Proof modification is discouraged.)
|- F/_ x A   &    |- F/_ x B   =>    |- F/ x A = B
Theorem	nfel 2629	Hypothesis builder for elementhood. (Contributed by NM, 1-Aug-1993.) (Revised by Mario Carneiro, 11-Aug-2016.) (Proof shortened by Wolf Lammen, 16-Nov-2019.)
|- F/_ x A   &    |- F/_ x B   =>    |- F/ x A e. B
Theorem	nfelOLD 2630	Obsolete proof of nfel 2629 as of 16-Nov-2019. (Contributed by NM, 1-Aug-1993.) (Revised by Mario Carneiro, 11-Aug-2016.) (New usage is discouraged.) (Proof modification is discouraged.)
|- F/_ x A   &    |- F/_ x B   =>    |- F/ x A e. B
Theorem	nfeq1 2631*	Hypothesis builder for equality, special case. (Contributed by Mario Carneiro, 10-Oct-2016.)
|- F/_ x A   =>    |- F/ x A = B
Theorem	nfel1 2632*	Hypothesis builder for elementhood, special case. (Contributed by Mario Carneiro, 10-Oct-2016.)
|- F/_ x A   =>    |- F/ x A e. B
Theorem	nfeq2 2633*	Hypothesis builder for equality, special case. (Contributed by Mario Carneiro, 10-Oct-2016.)
|- F/_ x B   =>    |- F/ x A = B
Theorem	nfel2 2634*	Hypothesis builder for elementhood, special case. (Contributed by Mario Carneiro, 10-Oct-2016.)
|- F/_ x B   =>    |- F/ x A e. B
Theorem	drnfc1 2635	Formula-building lemma for use with the Distinctor Reduction Theorem. (Contributed by Mario Carneiro, 8-Oct-2016.)
|- ( A. x x = y -> A = B )   =>    |- ( A. x x = y -> ( F/_ x A <-> F/_ y B ) )
Theorem	drnfc2 2636	Formula-building lemma for use with the Distinctor Reduction Theorem. (Contributed by Mario Carneiro, 8-Oct-2016.)
|- ( A. x x = y -> A = B )   =>    |- ( A. x x = y -> ( F/_ z A <-> F/_ z B ) )
Theorem	nfabd2 2637	Bound-variable hypothesis builder for a class abstraction. (Contributed by Mario Carneiro, 8-Oct-2016.)
|- F/ y ph   &    |- ( ( ph /\ -. A. x x = y ) -> F/ x ps )   =>    |- ( ph -> F/_ x { y | ps } )
Theorem	nfabd 2638	Bound-variable hypothesis builder for a class abstraction. (Contributed by Mario Carneiro, 8-Oct-2016.)
|- F/ y ph   &    |- ( ph -> F/ x ps )   =>    |- ( ph -> F/_ x { y | ps } )
Theorem	dvelimdc 2639	Deduction form of dvelimc 2640. (Contributed by Mario Carneiro, 8-Oct-2016.)
|- F/ x ph   &    |- F/ z ph   &    |- ( ph -> F/_ x A )   &    |- ( ph -> F/_ z B )   &    |- ( ph -> ( z = y -> A = B ) )   =>    |- ( ph -> ( -. A. x x = y -> F/_ x B ) )
Theorem	dvelimc 2640	Version of dvelim 2083 for classes. (Contributed by Mario Carneiro, 8-Oct-2016.)
|- F/_ x A   &    |- F/_ z B   &    |- ( z = y -> A = B )   =>    |- ( -. A. x x = y -> F/_ x B )
Theorem	nfcvf 2641	If x and y are distinct, then x is not free in y. (Contributed by Mario Carneiro, 8-Oct-2016.)
|- ( -. A. x x = y -> F/_ x y )
Theorem	nfcvf2 2642	If x and y are distinct, then y is not free in x. (Contributed by Mario Carneiro, 5-Dec-2016.)
|- ( -. A. x x = y -> F/_ y x )
Theorem	cleqf 2643	Establish equality between classes, using bound-variable hypotheses instead of distinct variable conditions. See also cleqh 2569. (Contributed by NM, 26-May-1993.) (Revised by Mario Carneiro, 7-Oct-2016.) (Proof shortened by Wolf Lammen, 17-Nov-2019.)
|- F/_ x A   &    |- F/_ x B   =>    |- ( A = B <-> A. x ( x e. A <-> x e. B ) )
Theorem	cleqfOLD 2644	Obsolete proof of cleqf 2643 as of 17-Nov-2019. (Contributed by NM, 26-May-1993.) (Revised by Mario Carneiro, 7-Oct-2016.) (Proof modification is discouraged.) (New usage is discouraged.)
|- F/_ x A   &    |- F/_ x B   =>    |- ( A = B <-> A. x ( x e. A <-> x e. B ) )
Theorem	abid2f 2645	A simplification of class abstraction. Theorem 5.2 of [Quine] p. 35. (Contributed by NM, 5-Sep-2011.) (Revised by Mario Carneiro, 7-Oct-2016.) (Proof shortened by Wolf Lammen, 17-Nov-2019.)
|- F/_ x A   =>    |- { x | x e. A } = A
Theorem	abid2fOLD 2646	Obsolete proof of abid2f 2645 as of 17-Nov-2019. (Contributed by NM, 5-Sep-2011.) (Revised by Mario Carneiro, 7-Oct-2016.) (Proof modification is discouraged.) (New usage is discouraged.)
|- F/_ x A   =>    |- { x | x e. A } = A
Theorem	sbabel 2647*	Theorem to move a substitution in and out of a class abstraction. (Contributed by NM, 27-Sep-2003.) (Revised by Mario Carneiro, 7-Oct-2016.) (Proof shortened by Wolf Lammen, 26-Dec-2019.)
|- F/_ x A   =>    |- ( [ y / x ] { z | ph } e. A <-> { z | [ y / x ] ph } e. A )
Theorem	sbabelOLD 2648*	Obsolete proof of sbabel 2647 as of 26-Dec-2019. (Contributed by NM, 27-Sep-2003.) (Revised by Mario Carneiro, 7-Oct-2016.) (Proof modification is discouraged.) (New usage is discouraged.)
|- F/_ x A   =>    |- ( [ y / x ] { z | ph } e. A <-> { z | [ y / x ] ph } e. A )
2.1.4  Negated equality and membership
Syntax	wne 2649	Extend wff notation to include inequality.
wff A =/= B
Syntax	wnel 2650	Extend wff notation to include negated membership.
wff A e/ B
Definition	df-ne 2651	Define inequality. (Contributed by NM, 26-May-1993.)
|- ( A =/= B <-> -. A = B )
Definition	df-nel 2652	Define negated membership. (Contributed by NM, 7-Aug-1994.)
|- ( A e/ B <-> -. A e. B )
2.1.4.1  Negated equality
Theorem	neii 2653	Inference associated with df-ne 2651. (Contributed by BJ, 7-Jul-2018.)
|- A =/= B   =>    |- -. A = B
Theorem	neir 2654	Inference associated with df-ne 2651. (Contributed by BJ, 7-Jul-2018.)
|- -. A = B   =>    |- A =/= B
Theorem	nne 2655	Negation of inequality. (Contributed by NM, 9-Jun-2006.)
|- ( -. A =/= B <-> A = B )
Theorem	neneqd 2656	Deduction eliminating inequality definition. (Contributed by Jonathan Ben-Naim, 3-Jun-2011.)
|- ( ph -> A =/= B )   =>    |- ( ph -> -. A = B )
Theorem	neqned 2657	If it is not the case that two classes are equal, they are unequal. Converse of neneqd 2656. One-way deduction form of df-ne 2651. (Contributed by David Moews, 28-Feb-2017.) Allow a shortening of necon3bi 2683. (Revised by Wolf Lammen, 22-Nov-2019.)
|- ( ph -> -. A = B )   =>    |- ( ph -> A =/= B )
Theorem	neirr 2658	No class is unequal to itself. Inequality is irreflexive. (Contributed by Stefan O'Rear, 1-Jan-2015.)
|- -. A =/= A
Theorem	exmidne 2659	Excluded middle with equality and inequality. (Contributed by NM, 3-Feb-2012.) (Proof shortened by Wolf Lammen, 17-Nov-2019.)
|- ( A = B \/ A =/= B )
Theorem	exmidneOLD 2660	Obsolete proof of exmidne 2659 as of 17-Nov-2019. (Contributed by NM, 3-Feb-2012.) (Proof modification is discouraged.) (New usage is discouraged.)
|- ( A = B \/ A =/= B )
Theorem	eqneqall 2661	A contradiction concerning equality implies anything. (Contributed by Alexander van der Vekens, 25-Jan-2018.)
|- ( A = B -> ( A =/= B -> ph ) )
Theorem	nonconne 2662	Law of noncontradiction with equality and inequality. (Contributed by NM, 3-Feb-2012.) (Proof shortened by Wolf Lammen, 21-Dec-2019.)
|- -. ( A = B /\ A =/= B )
Theorem	nonconneOLD 2663	Obsolete proof of nonconne 2662 as of 21-Dec-2019. (Contributed by NM, 3-Feb-2012.) (Proof modification is discouraged.) (New usage is discouraged.)
|- -. ( A = B /\ A =/= B )
Theorem	necon3ad 2664	Contrapositive law deduction for inequality. (Contributed by NM, 2-Apr-2007.) (Proof shortened by Andrew Salmon, 25-May-2011.) (Proof shortened by Wolf Lammen, 23-Nov-2019.)
|- ( ph -> ( ps -> A = B ) )   =>    |- ( ph -> ( A =/= B -> -. ps ) )
Theorem	necon3adOLD 2665	Obsolete proof of necon3ad 2664 as of 23-Nov-2019. (Contributed by NM, 2-Apr-2007.) (Proof shortened by Andrew Salmon, 25-May-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
|- ( ph -> ( ps -> A = B ) )   =>    |- ( ph -> ( A =/= B -> -. ps ) )
Theorem	necon3bd 2666	Contrapositive law deduction for inequality. (Contributed by NM, 2-Apr-2007.) (Proof shortened by Andrew Salmon, 25-May-2011.)
|- ( ph -> ( A = B -> ps ) )   =>    |- ( ph -> ( -. ps -> A =/= B ) )
Theorem	necon2ad 2667	Contrapositive inference for inequality. (Contributed by NM, 19-Apr-2007.) (Proof shortened by Andrew Salmon, 25-May-2011.) (Proof shortened by Wolf Lammen, 23-Nov-2019.)
|- ( ph -> ( A = B -> -. ps ) )   =>    |- ( ph -> ( ps -> A =/= B ) )
Theorem	necon2adOLD 2668	Obsolete proof of necon2ad 2667 as of 23-Nov-2019. (Contributed by NM, 19-Apr-2007.) (Proof shortened by Andrew Salmon, 25-May-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
|- ( ph -> ( A = B -> -. ps ) )   =>    |- ( ph -> ( ps -> A =/= B ) )
Theorem	necon2bd 2669	Contrapositive inference for inequality. (Contributed by NM, 13-Apr-2007.)
|- ( ph -> ( ps -> A =/= B ) )   =>    |- ( ph -> ( A = B -> -. ps ) )
Theorem	necon1ad 2670	Contrapositive deduction for inequality. (Contributed by NM, 2-Apr-2007.) (Proof shortened by Wolf Lammen, 23-Nov-2019.)
|- ( ph -> ( -. ps -> A = B ) )   =>    |- ( ph -> ( A =/= B -> ps ) )
Theorem	necon1adOLD 2671	Obsolete proof of necon1ad 2670 as of 23-Nov-2019. (Contributed by NM, 2-Apr-2007.) (Proof modification is discouraged.) (New usage is discouraged.)
|- ( ph -> ( -. ps -> A = B ) )   =>    |- ( ph -> ( A =/= B -> ps ) )
Theorem	necon1bd 2672	Contrapositive deduction for inequality. (Contributed by NM, 21-Mar-2007.) (Proof shortened by Andrew Salmon, 25-May-2011.) (Proof shortened by Wolf Lammen, 23-Nov-2019.)
|- ( ph -> ( A =/= B -> ps ) )   =>    |- ( ph -> ( -. ps -> A = B ) )
Theorem	necon1bdOLD 2673	Obsolete proof of necon1bd 2672 as of 23-Nov-2019. (Contributed by NM, 21-Mar-2007.) (Proof shortened by Andrew Salmon, 25-May-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
|- ( ph -> ( A =/= B -> ps ) )   =>    |- ( ph -> ( -. ps -> A = B ) )
Theorem	necon4ad 2674	Contrapositive inference for inequality. (Contributed by NM, 2-Apr-2007.) (Proof shortened by Andrew Salmon, 25-May-2011.) (Proof shortened by Wolf Lammen, 23-Nov-2019.)
|- ( ph -> ( A =/= B -> -. ps ) )   =>    |- ( ph -> ( ps -> A = B ) )
Theorem	necon4adOLD 2675	Obsolete proof of necon4ad 2674 as of 23-Nov-2019. (Contributed by NM, 2-Apr-2007.) (Proof shortened by Andrew Salmon, 25-May-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
|- ( ph -> ( A =/= B -> -. ps ) )   =>    |- ( ph -> ( ps -> A = B ) )
Theorem	necon4bd 2676	Contrapositive inference for inequality. (Contributed by NM, 1-Jun-2007.) (Proof shortened by Andrew Salmon, 25-May-2011.) (Proof shortened by Wolf Lammen, 23-Nov-2019.)
|- ( ph -> ( -. ps -> A =/= B ) )   =>    |- ( ph -> ( A = B -> ps ) )
Theorem	necon4bdOLD 2677	Obsolete proof of necon4bd 2676 as of 23-Nov-2019. (Contributed by NM, 1-Jun-2007.) (Proof shortened by Andrew Salmon, 25-May-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
|- ( ph -> ( -. ps -> A =/= B ) )   =>    |- ( ph -> ( A = B -> ps ) )
Theorem	necon3d 2678	Contrapositive law deduction for inequality. (Contributed by NM, 10-Jun-2006.)
|- ( ph -> ( A = B -> C = D ) )   =>    |- ( ph -> ( C =/= D -> A =/= B ) )
Theorem	necon1d 2679	Contrapositive law deduction for inequality. (Contributed by NM, 28-Dec-2008.) (Proof shortened by Andrew Salmon, 25-May-2011.)
|- ( ph -> ( A =/= B -> C = D ) )   =>    |- ( ph -> ( C =/= D -> A = B ) )
Theorem	necon2d 2680	Contrapositive inference for inequality. (Contributed by NM, 28-Dec-2008.)
|- ( ph -> ( A = B -> C =/= D ) )   =>    |- ( ph -> ( C = D -> A =/= B ) )
Theorem	necon4d 2681	Contrapositive inference for inequality. (Contributed by NM, 2-Apr-2007.) (Proof shortened by Andrew Salmon, 25-May-2011.)
|- ( ph -> ( A =/= B -> C =/= D ) )   =>    |- ( ph -> ( C = D -> A = B ) )
Theorem	necon3ai 2682	Contrapositive inference for inequality. (Contributed by NM, 23-May-2007.) (Proof shortened by Andrew Salmon, 25-May-2011.)
|- ( ph -> A = B )   =>    |- ( A =/= B -> -. ph )
Theorem	necon3bi 2683	Contrapositive inference for inequality. (Contributed by NM, 1-Jun-2007.) (Proof shortened by Andrew Salmon, 25-May-2011.) (Proof shortened by Wolf Lammen, 22-Nov-2019.)
|- ( A = B -> ph )   =>    |- ( -. ph -> A =/= B )
Theorem	necon3biOLD 2684	Obsolete proof of necon3bi 2683 as of 22-Nov-2019. (Contributed by NM, 1-Jun-2007.) (Proof shortened by Andrew Salmon, 25-May-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
|- ( A = B -> ph )   =>    |- ( -. ph -> A =/= B )
Theorem	necon1ai 2685	Contrapositive inference for inequality. (Contributed by NM, 12-Feb-2007.) (Proof shortened by Wolf Lammen, 22-Nov-2019.)
|- ( -. ph -> A = B )   =>    |- ( A =/= B -> ph )
Theorem	necon1aiOLD 2686	Obsolete proof of necon1ai 2685 as of 22-Nov-2019. (Contributed by NM, 12-Feb-2007.) (Proof modification is discouraged.) (New usage is discouraged.)
|- ( -. ph -> A = B )   =>    |- ( A =/= B -> ph )
Theorem	necon1bi 2687	Contrapositive inference for inequality. (Contributed by NM, 18-Mar-2007.) (Proof shortened by Andrew Salmon, 25-May-2011.) (Proof shortened by Wolf Lammen, 22-Nov-2019.)
|- ( A =/= B -> ph )   =>    |- ( -. ph -> A = B )
Theorem	necon1biOLD 2688	Obsolete proof of necon1bi 2687 as of 22-Nov-2019. (Contributed by NM, 18-Mar-2007.) (Proof shortened by Andrew Salmon, 25-May-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
|- ( A =/= B -> ph )   =>    |- ( -. ph -> A = B )
Theorem	necon2ai 2689	Contrapositive inference for inequality. (Contributed by NM, 16-Jan-2007.) (Proof shortened by Andrew Salmon, 25-May-2011.) (Proof shortened by Wolf Lammen, 22-Nov-2019.)
|- ( A = B -> -. ph )   =>    |- ( ph -> A =/= B )
Theorem	necon2aiOLD 2690	Obsolete proof of necon2ai 2689 as of 22-Nov-2019. (Contributed by NM, 16-Jan-2007.) (Proof shortened by Andrew Salmon, 25-May-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
|- ( A = B -> -. ph )   =>    |- ( ph -> A =/= B )
Theorem	necon2bi 2691	Contrapositive inference for inequality. (Contributed by NM, 1-Apr-2007.)
|- ( ph -> A =/= B )   =>    |- ( A = B -> -. ph )
Theorem	necon4ai 2692	Contrapositive inference for inequality. (Contributed by NM, 16-Jan-2007.) (Proof shortened by Andrew Salmon, 25-May-2011.) (Proof shortened by Wolf Lammen, 22-Nov-2019.)
|- ( A =/= B -> -. ph )   =>    |- ( ph -> A = B )
Theorem	necon4aiOLD 2693	Obsolete proof of necon4ai 2692 as of 22-Nov-2019. (Contributed by NM, 16-Jan-2007.) (Proof shortened by Andrew Salmon, 25-May-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
|- ( A =/= B -> -. ph )   =>    |- ( ph -> A = B )
Theorem	necon3i 2694	Contrapositive inference for inequality. (Contributed by NM, 9-Aug-2006.) (Proof shortened by Wolf Lammen, 22-Nov-2019.)
|- ( A = B -> C = D )   =>    |- ( C =/= D -> A =/= B )
Theorem	necon3iOLD 2695	Obsolete proof of necon3i 2694 as of 22-Nov-2019. (Contributed by NM, 9-Aug-2006.) (Proof modification is discouraged.) (New usage is discouraged.)
|- ( A = B -> C = D )   =>    |- ( C =/= D -> A =/= B )
Theorem	necon1i 2696	Contrapositive inference for inequality. (Contributed by NM, 18-Mar-2007.)
|- ( A =/= B -> C = D )   =>    |- ( C =/= D -> A = B )
Theorem	necon2i 2697	Contrapositive inference for inequality. (Contributed by NM, 18-Mar-2007.)
|- ( A = B -> C =/= D )   =>    |- ( C = D -> A =/= B )
Theorem	necon4i 2698	Contrapositive inference for inequality. (Contributed by NM, 17-Mar-2007.) (Proof shortened by Andrew Salmon, 25-May-2011.) (Proof shortened by Wolf Lammen, 24-Nov-2019.)
|- ( A =/= B -> C =/= D )   =>    |- ( C = D -> A = B )
Theorem	necon4iOLD 2699	Obsolete proof of necon4i 2698 as of 24-Nov-2019. (Contributed by NM, 17-Mar-2007.) (Proof shortened by Andrew Salmon, 25-May-2011.) (Proof modification is discouraged.) (New usage is discouraged.)
|- ( A =/= B -> C =/= D )   =>    |- ( C = D -> A = B )
Theorem	necon3abid 2700	Deduction from equality to inequality. (Contributed by NM, 21-Mar-2007.)
|- ( ph -> ( A = B <-> ps ) )   =>    |- ( ph -> ( A =/= B <-> -. ps ) )