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Type | Label | Description |
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Statement | ||
Theorem | bdssexg 15401 | Bounded version of ssexg 4168. (Contributed by BJ, 13-Nov-2019.) (Proof modification is discouraged.) |
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Theorem | bdssexd 15402 | Bounded version of ssexd 4169. (Contributed by BJ, 13-Nov-2019.) (Proof modification is discouraged.) |
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Theorem | bdrabexg 15403* | Bounded version of rabexg 4172. (Contributed by BJ, 19-Nov-2019.) (Proof modification is discouraged.) |
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Theorem | bj-inex 15404 | The intersection of two sets is a set, from bounded separation. (Contributed by BJ, 19-Nov-2019.) (Proof modification is discouraged.) |
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Theorem | bj-intexr 15405 | intexr 4179 from bounded separation. (Contributed by BJ, 18-Nov-2019.) (Proof modification is discouraged.) |
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Theorem | bj-intnexr 15406 | intnexr 4180 from bounded separation. (Contributed by BJ, 18-Nov-2019.) (Proof modification is discouraged.) |
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Theorem | bj-zfpair2 15407 | Proof of zfpair2 4239 using only bounded separation. (Contributed by BJ, 5-Oct-2019.) (Proof modification is discouraged.) |
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Theorem | bj-prexg 15408 | Proof of prexg 4240 using only bounded separation. (Contributed by BJ, 5-Oct-2019.) (Proof modification is discouraged.) |
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Theorem | bj-snexg 15409 | snexg 4213 from bounded separation. (Contributed by BJ, 5-Oct-2019.) (Proof modification is discouraged.) |
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Theorem | bj-snex 15410 | snex 4214 from bounded separation. (Contributed by BJ, 5-Oct-2019.) (Proof modification is discouraged.) |
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Theorem | bj-sels 15411* | If a class is a set, then it is a member of a set. (Copied from set.mm.) (Contributed by BJ, 3-Apr-2019.) |
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Theorem | bj-axun2 15412* | axun2 4466 from bounded separation. (Contributed by BJ, 15-Oct-2019.) (Proof modification is discouraged.) |
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Theorem | bj-uniex2 15413* | uniex2 4467 from bounded separation. (Contributed by BJ, 15-Oct-2019.) (Proof modification is discouraged.) |
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Theorem | bj-uniex 15414 | uniex 4468 from bounded separation. (Contributed by BJ, 13-Nov-2019.) (Proof modification is discouraged.) |
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Theorem | bj-uniexg 15415 | uniexg 4470 from bounded separation. (Contributed by BJ, 13-Nov-2019.) (Proof modification is discouraged.) |
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Theorem | bj-unex 15416 | unex 4472 from bounded separation. (Contributed by BJ, 13-Nov-2019.) (Proof modification is discouraged.) |
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Theorem | bdunexb 15417 | Bounded version of unexb 4473. (Contributed by BJ, 13-Nov-2019.) (Proof modification is discouraged.) |
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Theorem | bj-unexg 15418 | unexg 4474 from bounded separation. (Contributed by BJ, 13-Nov-2019.) (Proof modification is discouraged.) |
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Theorem | bj-sucexg 15419 | sucexg 4530 from bounded separation. (Contributed by BJ, 13-Nov-2019.) (Proof modification is discouraged.) |
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Theorem | bj-sucex 15420 | sucex 4531 from bounded separation. (Contributed by BJ, 13-Nov-2019.) (Proof modification is discouraged.) |
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Axiom | ax-bj-d0cl 15421 | Axiom for Δ0-classical logic. (Contributed by BJ, 2-Jan-2020.) |
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Theorem | bj-d0clsepcl 15422 | Δ0-classical logic and separation implies classical logic. (Contributed by BJ, 2-Jan-2020.) (Proof modification is discouraged.) |
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Syntax | wind 15423 | Syntax for inductive classes. |
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Definition | df-bj-ind 15424* | Define the property of being an inductive class. (Contributed by BJ, 30-Nov-2019.) |
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Theorem | bj-indsuc 15425 | A direct consequence of the definition of Ind. (Contributed by BJ, 30-Nov-2019.) |
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Theorem | bj-indeq 15426 | Equality property for Ind. (Contributed by BJ, 30-Nov-2019.) |
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Theorem | bj-bdind 15427 |
Boundedness of the formula "the setvar ![]() |
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Theorem | bj-indint 15428* | The property of being an inductive class is closed under intersections. (Contributed by BJ, 30-Nov-2019.) |
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Theorem | bj-indind 15429* |
If ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
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Theorem | bj-dfom 15430 |
Alternate definition of ![]() |
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Theorem | bj-omind 15431 |
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Theorem | bj-omssind 15432 |
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Theorem | bj-ssom 15433* |
A characterization of subclasses of ![]() |
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Theorem | bj-om 15434* |
A set is equal to ![]() |
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Theorem | bj-2inf 15435* | Two formulations of the axiom of infinity (see ax-infvn 15438 and bj-omex 15439) . (Contributed by BJ, 30-Nov-2019.) (Proof modification is discouraged.) |
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The first three Peano postulates follow from constructive set theory (actually, from its core axioms). The proofs peano1 4626 and peano3 4628 already show this. In this section, we prove bj-peano2 15436 to complete this program. We also prove a preliminary version of the fifth Peano postulate from the core axioms. | ||
Theorem | bj-peano2 15436 | Constructive proof of peano2 4627. Temporary note: another possibility is to simply replace sucexg 4530 with bj-sucexg 15419 in the proof of peano2 4627. (Contributed by BJ, 18-Nov-2019.) (Proof modification is discouraged.) |
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Theorem | peano5set 15437* |
Version of peano5 4630 when ![]() ![]() ![]() |
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In the absence of full separation, the axiom of infinity has to be stated more precisely, as the existence of the smallest class containing the empty set and the successor of each of its elements. | ||
In this section, we introduce the axiom of infinity in a constructive setting
(ax-infvn 15438) and deduce that the class | ||
Axiom | ax-infvn 15438* | Axiom of infinity in a constructive setting. This asserts the existence of the special set we want (the set of natural numbers), instead of the existence of a set with some properties (ax-iinf 4620) from which one then proves, using full separation, that the wanted set exists (omex 4625). "vn" is for "von Neumann". (Contributed by BJ, 14-Nov-2019.) |
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Theorem | bj-omex 15439 | Proof of omex 4625 from ax-infvn 15438. (Contributed by BJ, 14-Nov-2019.) (Proof modification is discouraged.) |
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In this section, we give constructive proofs of two versions of Peano's fifth postulate. | ||
Theorem | bdpeano5 15440* | Bounded version of peano5 4630. (Contributed by BJ, 19-Nov-2019.) (Proof modification is discouraged.) |
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Theorem | speano5 15441* |
Version of peano5 4630 when ![]() |
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In this section, we prove various versions of bounded induction from the basic axioms of CZF (in particular, without the axiom of set induction). We also prove Peano's fourth postulate. Together with the results from the previous sections, this proves from the core axioms of CZF (with infinity) that the set of natural number ordinals satisfies the five Peano postulates and thus provides a model for the set of natural numbers. | ||
Theorem | findset 15442* |
Bounded induction (principle of induction when ![]() ![]() |
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Theorem | bdfind 15443* |
Bounded induction (principle of induction when ![]() ![]() |
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Theorem | bj-bdfindis 15444* | Bounded induction (principle of induction for bounded formulas), using implicit substitutions (the biconditional versions of the hypotheses are implicit substitutions, and we have weakened them to implications). Constructive proof (from CZF). See finds 4632 for a proof of full induction in IZF. From this version, it is easy to prove bounded versions of finds 4632, finds2 4633, finds1 4634. (Contributed by BJ, 21-Nov-2019.) (Proof modification is discouraged.) |
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Theorem | bj-bdfindisg 15445* | Version of bj-bdfindis 15444 using a class term in the consequent. Constructive proof (from CZF). See the comment of bj-bdfindis 15444 for explanations. (Contributed by BJ, 21-Nov-2019.) (Proof modification is discouraged.) |
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Theorem | bj-bdfindes 15446 | Bounded induction (principle of induction for bounded formulas), using explicit substitutions. Constructive proof (from CZF). See the comment of bj-bdfindis 15444 for explanations. From this version, it is easy to prove the bounded version of findes 4635. (Contributed by BJ, 21-Nov-2019.) (Proof modification is discouraged.) |
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Theorem | bj-nn0suc0 15447* | Constructive proof of a variant of nn0suc 4636. For a constructive proof of nn0suc 4636, see bj-nn0suc 15461. (Contributed by BJ, 19-Nov-2019.) (Proof modification is discouraged.) |
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Theorem | bj-nntrans 15448 | A natural number is a transitive set. (Contributed by BJ, 22-Nov-2019.) (Proof modification is discouraged.) |
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Theorem | bj-nntrans2 15449 | A natural number is a transitive set. (Contributed by BJ, 22-Nov-2019.) (Proof modification is discouraged.) |
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Theorem | bj-nnelirr 15450 | A natural number does not belong to itself. Version of elirr 4573 for natural numbers, which does not require ax-setind 4569. (Contributed by BJ, 24-Nov-2019.) (Proof modification is discouraged.) |
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Theorem | bj-nnen2lp 15451 |
A version of en2lp 4586 for natural numbers, which does not require
ax-setind 4569.
Note: using this theorem and bj-nnelirr 15450, one can remove dependency on ax-setind 4569 from nntri2 6547 and nndcel 6553; one can actually remove more dependencies from these. (Contributed by BJ, 28-Nov-2019.) (Proof modification is discouraged.) |
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Theorem | bj-peano4 15452 | Remove from peano4 4629 dependency on ax-setind 4569. Therefore, it only requires core constructive axioms (albeit more of them). (Contributed by BJ, 28-Nov-2019.) (Proof modification is discouraged.) |
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Theorem | bj-omtrans 15453 |
The set ![]() ![]()
The idea is to use bounded induction with the formula |
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Theorem | bj-omtrans2 15454 |
The set ![]() |
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Theorem | bj-nnord 15455 | A natural number is an ordinal class. Constructive proof of nnord 4644. Can also be proved from bj-nnelon 15456 if the latter is proved from bj-omssonALT 15460. (Contributed by BJ, 27-Oct-2020.) (Proof modification is discouraged.) |
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Theorem | bj-nnelon 15456 | A natural number is an ordinal. Constructive proof of nnon 4642. Can also be proved from bj-omssonALT 15460. (Contributed by BJ, 27-Oct-2020.) (Proof modification is discouraged.) |
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Theorem | bj-omord 15457 |
The set ![]() |
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Theorem | bj-omelon 15458 |
The set ![]() |
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Theorem | bj-omsson 15459 | Constructive proof of omsson 4645. See also bj-omssonALT 15460. (Contributed by BJ, 27-Oct-2020.) (Proof modification is discouraged.) (New usage is discouraged. |
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Theorem | bj-omssonALT 15460 | Alternate proof of bj-omsson 15459. (Contributed by BJ, 27-Oct-2020.) (Proof modification is discouraged.) (New usage is discouraged.) |
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Theorem | bj-nn0suc 15461* |
Proof of (biconditional form of) nn0suc 4636 from the core axioms of CZF.
See also bj-nn0sucALT 15475. As a characterization of the elements of
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In this section, we add the axiom of set induction to the core axioms of CZF. | ||
In this section, we prove some variants of the axiom of set induction. | ||
Theorem | setindft 15462* | Axiom of set-induction with a disjoint variable condition replaced with a nonfreeness hypothesis. (Contributed by BJ, 22-Nov-2019.) |
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Theorem | setindf 15463* | Axiom of set-induction with a disjoint variable condition replaced with a nonfreeness hypothesis. (Contributed by BJ, 22-Nov-2019.) |
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Theorem | setindis 15464* | Axiom of set induction using implicit substitutions. (Contributed by BJ, 22-Nov-2019.) |
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Axiom | ax-bdsetind 15465* | Axiom of bounded set induction. (Contributed by BJ, 28-Nov-2019.) |
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Theorem | bdsetindis 15466* | Axiom of bounded set induction using implicit substitutions. (Contributed by BJ, 22-Nov-2019.) (Proof modification is discouraged.) |
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Theorem | bj-inf2vnlem1 15467* | Lemma for bj-inf2vn 15471. Remark: unoptimized proof (have to use more deduction style). (Contributed by BJ, 8-Dec-2019.) (Proof modification is discouraged.) |
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Theorem | bj-inf2vnlem2 15468* | Lemma for bj-inf2vnlem3 15469 and bj-inf2vnlem4 15470. Remark: unoptimized proof (have to use more deduction style). (Contributed by BJ, 8-Dec-2019.) (Proof modification is discouraged.) |
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Theorem | bj-inf2vnlem3 15469* | Lemma for bj-inf2vn 15471. (Contributed by BJ, 8-Dec-2019.) (Proof modification is discouraged.) |
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Theorem | bj-inf2vnlem4 15470* | Lemma for bj-inf2vn2 15472. (Contributed by BJ, 8-Dec-2019.) (Proof modification is discouraged.) |
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Theorem | bj-inf2vn 15471* |
A sufficient condition for ![]() |
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Theorem | bj-inf2vn2 15472* |
A sufficient condition for ![]() |
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Axiom | ax-inf2 15473* | Another axiom of infinity in a constructive setting (see ax-infvn 15438). (Contributed by BJ, 14-Nov-2019.) (New usage is discouraged.) |
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Theorem | bj-omex2 15474 |
Using bounded set induction and the strong axiom of infinity, ![]() |
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Theorem | bj-nn0sucALT 15475* | Alternate proof of bj-nn0suc 15461, also constructive but from ax-inf2 15473, hence requiring ax-bdsetind 15465. (Contributed by BJ, 8-Dec-2019.) (Proof modification is discouraged.) (New usage is discouraged.) |
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In this section, using the axiom of set induction, we prove full induction on the set of natural numbers. | ||
Theorem | bj-findis 15476* | Principle of induction, using implicit substitutions (the biconditional versions of the hypotheses are implicit substitutions, and we have weakened them to implications). Constructive proof (from CZF). See bj-bdfindis 15444 for a bounded version not requiring ax-setind 4569. See finds 4632 for a proof in IZF. From this version, it is easy to prove of finds 4632, finds2 4633, finds1 4634. (Contributed by BJ, 22-Dec-2019.) (Proof modification is discouraged.) |
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Theorem | bj-findisg 15477* | Version of bj-findis 15476 using a class term in the consequent. Constructive proof (from CZF). See the comment of bj-findis 15476 for explanations. (Contributed by BJ, 21-Nov-2019.) (Proof modification is discouraged.) |
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Theorem | bj-findes 15478 | Principle of induction, using explicit substitutions. Constructive proof (from CZF). See the comment of bj-findis 15476 for explanations. From this version, it is easy to prove findes 4635. (Contributed by BJ, 21-Nov-2019.) (Proof modification is discouraged.) |
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In this section, we state the axiom scheme of strong collection, which is part of CZF set theory. | ||
Axiom | ax-strcoll 15479* |
Axiom scheme of strong collection. It is stated with all possible
disjoint variable conditions, to show that this weak form is sufficient.
The antecedent means that ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
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Theorem | strcoll2 15480* | Version of ax-strcoll 15479 with one disjoint variable condition removed and without initial universal quantifier. (Contributed by BJ, 5-Oct-2019.) |
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Theorem | strcollnft 15481* | Closed form of strcollnf 15482. (Contributed by BJ, 21-Oct-2019.) |
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Theorem | strcollnf 15482* |
Version of ax-strcoll 15479 with one disjoint variable condition
removed,
the other disjoint variable condition replaced with a nonfreeness
hypothesis, and without initial universal quantifier. Version of
strcoll2 15480 with the disjoint variable condition on ![]() ![]() ![]()
This proof aims to demonstrate a standard technique, but strcoll2 15480 will
generally suffice: since the theorem asserts the existence of a set
|
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Theorem | strcollnfALT 15483* | Alternate proof of strcollnf 15482, not using strcollnft 15481. (Contributed by BJ, 5-Oct-2019.) (Proof modification is discouraged.) (New usage is discouraged.) |
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In this section, we state the axiom scheme of subset collection, which is part of CZF set theory. | ||
Axiom | ax-sscoll 15484* |
Axiom scheme of subset collection. It is stated with all possible
disjoint variable conditions, to show that this weak form is sufficient.
The antecedent means that ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
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Theorem | sscoll2 15485* | Version of ax-sscoll 15484 with two disjoint variable conditions removed and without initial universal quantifiers. (Contributed by BJ, 5-Oct-2019.) |
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Axiom | ax-ddkcomp 15486 | Axiom of Dedekind completeness for Dedekind real numbers: every inhabited upper-bounded located set of reals has a real upper bound. Ideally, this axiom should be "proved" as "axddkcomp" for the real numbers constructed from IZF, and then Axiom ax-ddkcomp 15486 should be used in place of construction specific results. In particular, axcaucvg 7960 should be proved from it. (Contributed by BJ, 24-Oct-2021.) |
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Theorem | nnnotnotr 15487 | Double negation of double negation elimination. Suggested by an online post by Martin Escardo. Although this statement resembles nnexmid 851, it can be proved with reference only to implication and negation (that is, without use of disjunction). (Contributed by Jim Kingdon, 21-Oct-2024.) |
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Theorem | 1dom1el 15488 | If a set is dominated by one, then any two of its elements are equal. (Contributed by Jim Kingdon, 23-Apr-2025.) |
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Theorem | ss1oel2o 15489 | Any subset of ordinal one being an element of ordinal two is equivalent to excluded middle. A variation of exmid01 4227 which more directly illustrates the contrast with el2oss1o 6496. (Contributed by Jim Kingdon, 8-Aug-2022.) |
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Theorem | nnti 15490 | Ordering on a natural number generates a tight apartness. (Contributed by Jim Kingdon, 7-Aug-2022.) |
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Theorem | 012of 15491 |
Mapping zero and one between ![]() ![]() |
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Theorem | 2o01f 15492 |
Mapping zero and one between ![]() ![]() |
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Theorem | pwtrufal 15493 |
A subset of the singleton ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
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Theorem | pwle2 15494* |
An exercise related to ![]() |
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Theorem | pwf1oexmid 15495* |
An exercise related to ![]() |
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Theorem | subctctexmid 15496* | If every subcountable set is countable and Markov's principle holds, excluded middle follows. Proposition 2.6 of [BauerSwan], p. 14:4. The proof is taken from that paper. (Contributed by Jim Kingdon, 29-Nov-2023.) |
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Theorem | sssneq 15497* | Any two elements of a subset of a singleton are equal. (Contributed by Jim Kingdon, 28-May-2024.) |
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Theorem | pw1nct 15498* | A condition which ensures that the powerset of a singleton is not countable. The antecedent here can be referred to as the uniformity principle. Based on Mastodon posts by Andrej Bauer and Rahul Chhabra. (Contributed by Jim Kingdon, 29-May-2024.) |
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Theorem | 0nninf 15499 |
The zero element of ℕ∞ (the constant sequence equal to
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Theorem | nnsf 15500* |
Domain and range of ![]() |
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