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Theorem List for Intuitionistic Logic Explorer - 2801-2900   *Has distinct variable group(s)
TypeLabelDescription
Statement

Theoremeueq1 2801* Equality has existential uniqueness. (Contributed by NM, 5-Apr-1995.)

Theoremeueq2dc 2802* Equality has existential uniqueness (split into 2 cases). (Contributed by NM, 5-Apr-1995.)
DECID

Theoremeueq3dc 2803* Equality has existential uniqueness (split into 3 cases). (Contributed by NM, 5-Apr-1995.) (Proof shortened by Mario Carneiro, 28-Sep-2015.)
DECID DECID

Theoremmoeq 2804* There is at most one set equal to a class. (Contributed by NM, 8-Mar-1995.)

Theoremmoeq3dc 2805* "At most one" property of equality (split into 3 cases). (Contributed by Jim Kingdon, 7-Jul-2018.)
DECID DECID

Theoremmosubt 2806* "At most one" remains true after substitution. (Contributed by Jim Kingdon, 18-Jan-2019.)

Theoremmosub 2807* "At most one" remains true after substitution. (Contributed by NM, 9-Mar-1995.)

Theoremmo2icl 2808* Theorem for inferring "at most one." (Contributed by NM, 17-Oct-1996.)

Theoremmob2 2809* Consequence of "at most one." (Contributed by NM, 2-Jan-2015.)

Theoremmoi2 2810* Consequence of "at most one." (Contributed by NM, 29-Jun-2008.)

Theoremmob 2811* Equality implied by "at most one." (Contributed by NM, 18-Feb-2006.)

Theoremmoi 2812* Equality implied by "at most one." (Contributed by NM, 18-Feb-2006.)

Theoremmorex 2813* Derive membership from uniqueness. (Contributed by Jeff Madsen, 2-Sep-2009.)

Theoremeuxfr2dc 2814* Transfer existential uniqueness from a variable to another variable contained in expression . (Contributed by NM, 14-Nov-2004.)
DECID

Theoremeuxfrdc 2815* Transfer existential uniqueness from a variable to another variable contained in expression . (Contributed by NM, 14-Nov-2004.)
DECID

Theoremeuind 2816* Existential uniqueness via an indirect equality. (Contributed by NM, 11-Oct-2010.)

Theoremreu2 2817* A way to express restricted uniqueness. (Contributed by NM, 22-Nov-1994.)

Theoremreu6 2818* A way to express restricted uniqueness. (Contributed by NM, 20-Oct-2006.)

Theoremreu3 2819* A way to express restricted uniqueness. (Contributed by NM, 24-Oct-2006.)

Theoremreu6i 2820* A condition which implies existential uniqueness. (Contributed by Mario Carneiro, 2-Oct-2015.)

Theoremeqreu 2821* A condition which implies existential uniqueness. (Contributed by Mario Carneiro, 2-Oct-2015.)

Theoremrmo4 2822* Restricted "at most one" using implicit substitution. (Contributed by NM, 24-Oct-2006.) (Revised by NM, 16-Jun-2017.)

Theoremreu4 2823* Restricted uniqueness using implicit substitution. (Contributed by NM, 23-Nov-1994.)

Theoremreu7 2824* Restricted uniqueness using implicit substitution. (Contributed by NM, 24-Oct-2006.)

Theoremreu8 2825* Restricted uniqueness using implicit substitution. (Contributed by NM, 24-Oct-2006.)

Theoremrmo3f 2826* Restricted "at most one" using explicit substitution. (Contributed by NM, 4-Nov-2012.) (Revised by NM, 16-Jun-2017.) (Revised by Thierry Arnoux, 8-Oct-2017.)

Theoremrmo4f 2827* Restricted "at most one" using implicit substitution. (Contributed by NM, 24-Oct-2006.) (Revised by Thierry Arnoux, 11-Oct-2016.) (Revised by Thierry Arnoux, 8-Mar-2017.) (Revised by Thierry Arnoux, 8-Oct-2017.)

Theoremreueq 2828* Equality has existential uniqueness. (Contributed by Mario Carneiro, 1-Sep-2015.)

Theoremrmoan 2829 Restricted "at most one" still holds when a conjunct is added. (Contributed by NM, 16-Jun-2017.)

Theoremrmoim 2830 Restricted "at most one" is preserved through implication (note wff reversal). (Contributed by Alexander van der Vekens, 17-Jun-2017.)

Theoremrmoimia 2831 Restricted "at most one" is preserved through implication (note wff reversal). (Contributed by Alexander van der Vekens, 17-Jun-2017.)

Theoremrmoimi2 2832 Restricted "at most one" is preserved through implication (note wff reversal). (Contributed by Alexander van der Vekens, 17-Jun-2017.)

Theorem2reuswapdc 2833* A condition allowing swap of uniqueness and existential quantifiers. (Contributed by Thierry Arnoux, 7-Apr-2017.) (Revised by NM, 16-Jun-2017.)
DECID

Theoremreuind 2834* Existential uniqueness via an indirect equality. (Contributed by NM, 16-Oct-2010.)

Theorem2rmorex 2835* Double restricted quantification with "at most one," analogous to 2moex 2041. (Contributed by Alexander van der Vekens, 17-Jun-2017.)

Theoremnelrdva 2836* Deduce negative membership from an implication. (Contributed by Thierry Arnoux, 27-Nov-2017.)

2.1.7  Conditional equality (experimental)

This is a very useless definition, which "abbreviates" as CondEq . What this display hides, though, is that the first expression, even though it has a shorter constant string, is actually much more complicated in its parse tree: it is parsed as (wi (wceq (cv vx) (cv vy)) wph), while the CondEq version is parsed as (wcdeq vx vy wph). It also allows us to give a name to the specific ternary operation .

This is all used as part of a metatheorem: we want to say that and are provable, for any expressions or in the language. The proof is by induction, so the base case is each of the primitives, which is why you will see a theorem for each of the set.mm primitive operations.

The metatheorem comes with a disjoint variables condition: every variable in is assumed disjoint from except itself. For such a proof by induction, we must consider each of the possible forms of . If it is a variable other than , then we have CondEq or CondEq , which is provable by cdeqth 2841 and reflexivity. Since we are only working with class and wff expressions, it can't be itself in set.mm, but if it was we'd have to also prove CondEq (where set equality is being used on the right).

Otherwise, it is a primitive operation applied to smaller expressions. In these cases, for each setvar variable parameter to the operation, we must consider if it is equal to or not, which yields 2^n proof obligations. Luckily, all primitive operations in set.mm have either zero or one set variable, so we only need to prove one statement for the non-set constructors (like implication) and two for the constructors taking a set (the forall and the class builder).

In each of the primitive proofs, we are allowed to assume that is disjoint from and vice versa, because this is maintained through the induction. This is how we satisfy the disjoint variable conditions of cdeqab1 2846 and cdeqab 2844.

Syntaxwcdeq 2837 Extend wff notation to include conditional equality. This is a technical device used in the proof that is the not-free predicate, and that definitions are conservative as a result.
CondEq

Definitiondf-cdeq 2838 Define conditional equality. All the notation to the left of the is fake; the parentheses and arrows are all part of the notation, which could equally well be written CondEq. On the right side is the actual implication arrow. The reason for this definition is to "flatten" the structure on the right side (whose tree structure is something like (wi (wceq (cv vx) (cv vy)) wph) ) into just (wcdeq vx vy wph). (Contributed by Mario Carneiro, 11-Aug-2016.)
CondEq

Theoremcdeqi 2839 Deduce conditional equality. (Contributed by Mario Carneiro, 11-Aug-2016.)
CondEq

Theoremcdeqri 2840 Property of conditional equality. (Contributed by Mario Carneiro, 11-Aug-2016.)
CondEq

Theoremcdeqth 2841 Deduce conditional equality from a theorem. (Contributed by Mario Carneiro, 11-Aug-2016.)
CondEq

Theoremcdeqnot 2842 Distribute conditional equality over negation. (Contributed by Mario Carneiro, 11-Aug-2016.)
CondEq        CondEq

Theoremcdeqal 2843* Distribute conditional equality over quantification. (Contributed by Mario Carneiro, 11-Aug-2016.)
CondEq        CondEq

Theoremcdeqab 2844* Distribute conditional equality over abstraction. (Contributed by Mario Carneiro, 11-Aug-2016.)
CondEq        CondEq

Theoremcdeqal1 2845* Distribute conditional equality over quantification. (Contributed by Mario Carneiro, 11-Aug-2016.)
CondEq        CondEq

Theoremcdeqab1 2846* Distribute conditional equality over abstraction. (Contributed by Mario Carneiro, 11-Aug-2016.)
CondEq        CondEq

Theoremcdeqim 2847 Distribute conditional equality over implication. (Contributed by Mario Carneiro, 11-Aug-2016.)
CondEq        CondEq        CondEq

Theoremcdeqcv 2848 Conditional equality for set-to-class promotion. (Contributed by Mario Carneiro, 11-Aug-2016.)
CondEq

Theoremcdeqeq 2849 Distribute conditional equality over equality. (Contributed by Mario Carneiro, 11-Aug-2016.)
CondEq        CondEq        CondEq

Theoremcdeqel 2850 Distribute conditional equality over elementhood. (Contributed by Mario Carneiro, 11-Aug-2016.)
CondEq        CondEq        CondEq

Theoremnfcdeq 2851* If we have a conditional equality proof, where is and is , and in fact does not have free in it according to , then unconditionally. This proves that is actually a not-free predicate. (Contributed by Mario Carneiro, 11-Aug-2016.)
CondEq

Theoremnfccdeq 2852* Variation of nfcdeq 2851 for classes. (Contributed by Mario Carneiro, 11-Aug-2016.)
CondEq

Theoremru 2853 Russell's Paradox. Proposition 4.14 of [TakeutiZaring] p. 14.

In the late 1800s, Frege's Axiom of (unrestricted) Comprehension, expressed in our notation as , asserted that any collection of sets is a set i.e. belongs to the universe of all sets. In particular, by substituting (the "Russell class") for , it asserted , meaning that the "collection of all sets which are not members of themselves" is a set. However, here we prove . This contradiction was discovered by Russell in 1901 (published in 1903), invalidating the Comprehension Axiom and leading to the collapse of Frege's system.

In 1908, Zermelo rectified this fatal flaw by replacing Comprehension with a weaker Subset (or Separation) Axiom asserting that is a set only when it is smaller than some other set . The intuitionistic set theory IZF includes such a separation axiom, Axiom 6 of [Crosilla] p. "Axioms of CZF and IZF", which we include as ax-sep 3978. (Contributed by NM, 7-Aug-1994.)

2.1.9  Proper substitution of classes for sets

Syntaxwsbc 2854 Extend wff notation to include the proper substitution of a class for a set. Read this notation as "the proper substitution of class for setvar variable in wff ."

Definitiondf-sbc 2855 Define the proper substitution of a class for a set.

When is a proper class, our definition evaluates to false. This is somewhat arbitrary: we could have, instead, chosen the conclusion of sbc6 2879 for our definition, which always evaluates to true for proper classes.

Our definition also does not produce the same results as discussed in the proof of Theorem 6.6 of [Quine] p. 42 (although Theorem 6.6 itself does hold, as shown by dfsbcq 2856 below). Unfortunately, Quine's definition requires a recursive syntactical breakdown of , and it does not seem possible to express it with a single closed formula.

If we did not want to commit to any specific proper class behavior, we could use this definition only to prove theorem dfsbcq 2856, which holds for both our definition and Quine's, and from which we can derive a weaker version of df-sbc 2855 in the form of sbc8g 2861. However, the behavior of Quine's definition at proper classes is similarly arbitrary, and for practical reasons (to avoid having to prove sethood of in every use of this definition) we allow direct reference to df-sbc 2855 and assert that is always false when is a proper class.

The related definition df-csb defines proper substitution into a class variable (as opposed to a wff variable). (Contributed by NM, 14-Apr-1995.) (Revised by NM, 25-Dec-2016.)

Theoremdfsbcq 2856 This theorem, which is similar to Theorem 6.7 of [Quine] p. 42 and holds under both our definition and Quine's, provides us with a weak definition of the proper substitution of a class for a set. Since our df-sbc 2855 does not result in the same behavior as Quine's for proper classes, if we wished to avoid conflict with Quine's definition we could start with this theorem and dfsbcq2 2857 instead of df-sbc 2855. (dfsbcq2 2857 is needed because unlike Quine we do not overload the df-sb 1700 syntax.) As a consequence of these theorems, we can derive sbc8g 2861, which is a weaker version of df-sbc 2855 that leaves substitution undefined when is a proper class.

However, it is often a nuisance to have to prove the sethood hypothesis of sbc8g 2861, so we will allow direct use of df-sbc 2855. Proper substiution with a proper class is rarely needed, and when it is, we can simply use the expansion of Quine's definition. (Contributed by NM, 14-Apr-1995.)

Theoremdfsbcq2 2857 This theorem, which is similar to Theorem 6.7 of [Quine] p. 42 and holds under both our definition and Quine's, relates logic substitution df-sb 1700 and substitution for class variables df-sbc 2855. Unlike Quine, we use a different syntax for each in order to avoid overloading it. See remarks in dfsbcq 2856. (Contributed by NM, 31-Dec-2016.)

Theoremsbsbc 2858 Show that df-sb 1700 and df-sbc 2855 are equivalent when the class term in df-sbc 2855 is a setvar variable. This theorem lets us reuse theorems based on df-sb 1700 for proofs involving df-sbc 2855. (Contributed by NM, 31-Dec-2016.) (Proof modification is discouraged.)

Theoremsbceq1d 2859 Equality theorem for class substitution. (Contributed by Mario Carneiro, 9-Feb-2017.) (Revised by NM, 30-Jun-2018.)

Theoremsbceq1dd 2860 Equality theorem for class substitution. (Contributed by Mario Carneiro, 9-Feb-2017.) (Revised by NM, 30-Jun-2018.)

Theoremsbc8g 2861 This is the closest we can get to df-sbc 2855 if we start from dfsbcq 2856 (see its comments) and dfsbcq2 2857. (Contributed by NM, 18-Nov-2008.) (Proof shortened by Andrew Salmon, 29-Jun-2011.) (Proof modification is discouraged.)

Theoremsbcex 2862 By our definition of proper substitution, it can only be true if the substituted expression is a set. (Contributed by Mario Carneiro, 13-Oct-2016.)

Theoremsbceq1a 2863 Equality theorem for class substitution. Class version of sbequ12 1708. (Contributed by NM, 26-Sep-2003.)

Theoremsbceq2a 2864 Equality theorem for class substitution. Class version of sbequ12r 1709. (Contributed by NM, 4-Jan-2017.)

Theoremspsbc 2865 Specialization: if a formula is true for all sets, it is true for any class which is a set. Similar to Theorem 6.11 of [Quine] p. 44. See also stdpc4 1712 and rspsbc 2935. (Contributed by NM, 16-Jan-2004.)

Theoremspsbcd 2866 Specialization: if a formula is true for all sets, it is true for any class which is a set. Similar to Theorem 6.11 of [Quine] p. 44. See also stdpc4 1712 and rspsbc 2935. (Contributed by Mario Carneiro, 9-Feb-2017.)

Theoremsbcth 2867 A substitution into a theorem remains true (when is a set). (Contributed by NM, 5-Nov-2005.)

Theoremsbcthdv 2868* Deduction version of sbcth 2867. (Contributed by NM, 30-Nov-2005.) (Proof shortened by Andrew Salmon, 8-Jun-2011.)

Theoremsbcid 2869 An identity theorem for substitution. See sbid 1711. (Contributed by Mario Carneiro, 18-Feb-2017.)

Theoremnfsbc1d 2870 Deduction version of nfsbc1 2871. (Contributed by NM, 23-May-2006.) (Revised by Mario Carneiro, 12-Oct-2016.)

Theoremnfsbc1 2871 Bound-variable hypothesis builder for class substitution. (Contributed by Mario Carneiro, 12-Oct-2016.)

Theoremnfsbc1v 2872* Bound-variable hypothesis builder for class substitution. (Contributed by Mario Carneiro, 12-Oct-2016.)

Theoremnfsbcd 2873 Deduction version of nfsbc 2874. (Contributed by NM, 23-Nov-2005.) (Revised by Mario Carneiro, 12-Oct-2016.)

Theoremnfsbc 2874 Bound-variable hypothesis builder for class substitution. (Contributed by NM, 7-Sep-2014.) (Revised by Mario Carneiro, 12-Oct-2016.)

Theoremsbcco 2875* A composition law for class substitution. (Contributed by NM, 26-Sep-2003.) (Revised by Mario Carneiro, 13-Oct-2016.)

Theoremsbcco2 2876* A composition law for class substitution. Importantly, may occur free in the class expression substituted for . (Contributed by NM, 5-Sep-2004.) (Proof shortened by Andrew Salmon, 8-Jun-2011.)

Theoremsbc5 2877* An equivalence for class substitution. (Contributed by NM, 23-Aug-1993.) (Revised by Mario Carneiro, 12-Oct-2016.)

Theoremsbc6g 2878* An equivalence for class substitution. (Contributed by NM, 11-Oct-2004.) (Proof shortened by Andrew Salmon, 8-Jun-2011.)

Theoremsbc6 2879* An equivalence for class substitution. (Contributed by NM, 23-Aug-1993.) (Proof shortened by Eric Schmidt, 17-Jan-2007.)

Theoremsbc7 2880* An equivalence for class substitution in the spirit of df-clab 2082. Note that and don't have to be distinct. (Contributed by NM, 18-Nov-2008.) (Revised by Mario Carneiro, 13-Oct-2016.)

Theoremcbvsbc 2881 Change bound variables in a wff substitution. (Contributed by Jeff Hankins, 19-Sep-2009.) (Proof shortened by Andrew Salmon, 8-Jun-2011.)

Theoremcbvsbcv 2882* Change the bound variable of a class substitution using implicit substitution. (Contributed by NM, 30-Sep-2008.) (Revised by Mario Carneiro, 13-Oct-2016.)

Theoremsbciegft 2883* Conversion of implicit substitution to explicit class substitution, using a bound-variable hypothesis instead of distinct variables. (Closed theorem version of sbciegf 2884.) (Contributed by NM, 10-Nov-2005.) (Revised by Mario Carneiro, 13-Oct-2016.)

Theoremsbciegf 2884* Conversion of implicit substitution to explicit class substitution. (Contributed by NM, 14-Dec-2005.) (Revised by Mario Carneiro, 13-Oct-2016.)

Theoremsbcieg 2885* Conversion of implicit substitution to explicit class substitution. (Contributed by NM, 10-Nov-2005.)

Theoremsbcie2g 2886* Conversion of implicit substitution to explicit class substitution. This version of sbcie 2887 avoids a disjointness condition on and by substituting twice. (Contributed by Mario Carneiro, 15-Oct-2016.)

Theoremsbcie 2887* Conversion of implicit substitution to explicit class substitution. (Contributed by NM, 4-Sep-2004.)

Theoremsbciedf 2888* Conversion of implicit substitution to explicit class substitution, deduction form. (Contributed by NM, 29-Dec-2014.)

Theoremsbcied 2889* Conversion of implicit substitution to explicit class substitution, deduction form. (Contributed by NM, 13-Dec-2014.)

Theoremsbcied2 2890* Conversion of implicit substitution to explicit class substitution, deduction form. (Contributed by NM, 13-Dec-2014.)

Theoremelrabsf 2891 Membership in a restricted class abstraction, expressed with explicit class substitution. (The variation elrabf 2783 has implicit substitution). The hypothesis specifies that must not be a free variable in . (Contributed by NM, 30-Sep-2003.) (Proof shortened by Mario Carneiro, 13-Oct-2016.)

Theoremeqsbc3 2892* Substitution applied to an atomic wff. Set theory version of eqsb3 2198. (Contributed by Andrew Salmon, 29-Jun-2011.)

Theoremsbcng 2893 Move negation in and out of class substitution. (Contributed by NM, 16-Jan-2004.)

Theoremsbcimg 2894 Distribution of class substitution over implication. (Contributed by NM, 16-Jan-2004.)

Theoremsbcan 2895 Distribution of class substitution over conjunction. (Contributed by NM, 31-Dec-2016.)

Theoremsbcang 2896 Distribution of class substitution over conjunction. (Contributed by NM, 21-May-2004.)

Theoremsbcor 2897 Distribution of class substitution over disjunction. (Contributed by NM, 31-Dec-2016.)

Theoremsbcorg 2898 Distribution of class substitution over disjunction. (Contributed by NM, 21-May-2004.)

Theoremsbcbig 2899 Distribution of class substitution over biconditional. (Contributed by Raph Levien, 10-Apr-2004.)

Theoremsbcn1 2900 Move negation in and out of class substitution. One direction of sbcng 2893 that holds for proper classes. (Contributed by NM, 17-Aug-2018.)

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