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Type | Label | Description |
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Statement | ||
Theorem | ordsucim 4501 | The successor of an ordinal class is ordinal. (Contributed by Jim Kingdon, 8-Nov-2018.) |
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Theorem | onsuc 4502 | The successor of an ordinal number is an ordinal number. Closed form of onsuci 4517. Forward implication of onsucb 4504. Proposition 7.24 of [TakeutiZaring] p. 41. (Contributed by NM, 6-Jun-1994.) |
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Theorem | ordsucg 4503 | The successor of an ordinal class is ordinal. (Contributed by Jim Kingdon, 20-Nov-2018.) |
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Theorem | onsucb 4504 | A class is an ordinal number if and only if its successor is an ordinal number. Biconditional form of onsuc 4502. (Contributed by NM, 9-Sep-2003.) |
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Theorem | ordsucss 4505 | The successor of an element of an ordinal class is a subset of it. (Contributed by NM, 21-Jun-1998.) |
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Theorem | ordelsuc 4506 | A set belongs to an ordinal iff its successor is a subset of the ordinal. Exercise 8 of [TakeutiZaring] p. 42 and its converse. (Contributed by NM, 29-Nov-2003.) |
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Theorem | onsucssi 4507 | A set belongs to an ordinal number iff its successor is a subset of the ordinal number. Exercise 8 of [TakeutiZaring] p. 42 and its converse. (Contributed by NM, 16-Sep-1995.) |
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Theorem | onsucmin 4508* | The successor of an ordinal number is the smallest larger ordinal number. (Contributed by NM, 28-Nov-2003.) |
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Theorem | onsucelsucr 4509 |
Membership is inherited by predecessors. The converse, for all ordinals,
implies excluded middle, as shown at onsucelsucexmid 4531. However, the
converse does hold where ![]() |
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Theorem | onsucsssucr 4510 | The subclass relationship between two ordinals is inherited by their predecessors. The converse implies excluded middle, as shown at onsucsssucexmid 4528. (Contributed by Mario Carneiro and Jim Kingdon, 29-Jul-2019.) |
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Theorem | sucunielr 4511 |
Successor and union. The converse (where ![]() |
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Theorem | unon 4512 | The class of all ordinal numbers is its own union. Exercise 11 of [TakeutiZaring] p. 40. (Contributed by NM, 12-Nov-2003.) |
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Theorem | onuniss2 4513* | The union of the ordinal subsets of an ordinal number is that number. (Contributed by Jim Kingdon, 2-Aug-2019.) |
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Theorem | limon 4514 | The class of ordinal numbers is a limit ordinal. (Contributed by NM, 24-Mar-1995.) |
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Theorem | ordunisuc2r 4515* | An ordinal which contains the successor of each of its members is equal to its union. (Contributed by Jim Kingdon, 14-Nov-2018.) |
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Theorem | onssi 4516 |
An ordinal number is a subset of ![]() |
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Theorem | onsuci 4517 | The successor of an ordinal number is an ordinal number. Inference associated with onsuc 4502 and onsucb 4504. Corollary 7N(c) of [Enderton] p. 193. (Contributed by NM, 12-Jun-1994.) |
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Theorem | onintonm 4518* | The intersection of an inhabited collection of ordinal numbers is an ordinal number. Compare Exercise 6 of [TakeutiZaring] p. 44. (Contributed by Mario Carneiro and Jim Kingdon, 30-Aug-2021.) |
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Theorem | onintrab2im 4519 | An existence condition which implies an intersection is an ordinal number. (Contributed by Jim Kingdon, 30-Aug-2021.) |
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Theorem | ordtriexmidlem 4520 |
Lemma for decidability and ordinals. The set ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
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Theorem | ordtriexmidlem2 4521* |
Lemma for decidability and ordinals. The set ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
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Theorem | ordtriexmid 4522* |
Ordinal trichotomy implies the law of the excluded middle (that is,
decidability of an arbitrary proposition).
This theorem is stated in "Constructive ordinals", [Crosilla], p. "Set-theoretic principles incompatible with intuitionistic logic". Also see exmidontri 7240 which is much the same theorem but biconditionalized and using the EXMID notation. (Contributed by Mario Carneiro and Jim Kingdon, 14-Nov-2018.) |
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Theorem | ontriexmidim 4523* | Ordinal trichotomy implies excluded middle. Closed form of ordtriexmid 4522. (Contributed by Jim Kingdon, 26-Aug-2024.) |
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Theorem | ordtri2orexmid 4524* | Ordinal trichotomy implies excluded middle. (Contributed by Jim Kingdon, 31-Jul-2019.) |
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Theorem | 2ordpr 4525 |
Version of 2on 6428 with the definition of ![]() ![]() |
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Theorem | ontr2exmid 4526* | An ordinal transitivity law which implies excluded middle. (Contributed by Jim Kingdon, 17-Sep-2021.) |
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Theorem | ordtri2or2exmidlem 4527* |
A set which is ![]() ![]() ![]() ![]() ![]() |
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Theorem | onsucsssucexmid 4528* | The converse of onsucsssucr 4510 implies excluded middle. (Contributed by Mario Carneiro and Jim Kingdon, 29-Jul-2019.) |
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Theorem | onsucelsucexmidlem1 4529* | Lemma for onsucelsucexmid 4531. (Contributed by Jim Kingdon, 2-Aug-2019.) |
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Theorem | onsucelsucexmidlem 4530* |
Lemma for onsucelsucexmid 4531. The set
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Theorem | onsucelsucexmid 4531* |
The converse of onsucelsucr 4509 implies excluded middle. On the other
hand, if ![]() |
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Theorem | ordsucunielexmid 4532* |
The converse of sucunielr 4511 (where ![]() |
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Theorem | regexmidlemm 4533* |
Lemma for regexmid 4536. ![]() |
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Theorem | regexmidlem1 4534* |
Lemma for regexmid 4536. If ![]() |
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Theorem | reg2exmidlema 4535* |
Lemma for reg2exmid 4537. If ![]() ![]() |
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Theorem | regexmid 4536* |
The axiom of foundation implies excluded middle.
By foundation (or regularity), we mean the principle that every
inhabited set has an element which is minimal (when arranged by
For this reason, IZF does not adopt foundation as an axiom and instead replaces it with ax-setind 4538. (Contributed by Jim Kingdon, 3-Sep-2019.) |
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Theorem | reg2exmid 4537* |
If any inhabited set has a minimal element (when expressed by ![]() |
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Axiom | ax-setind 4538* |
Axiom of ![]() For more on axioms which might be adopted which are incompatible with this axiom (that is, Non-wellfounded Set Theory but in the absence of excluded middle), see Chapter 20 of [AczelRathjen], p. 183. (Contributed by Jim Kingdon, 19-Oct-2018.) |
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Theorem | setindel 4539* |
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Theorem | setind 4540* | Set (epsilon) induction. Theorem 5.22 of [TakeutiZaring] p. 21. (Contributed by NM, 17-Sep-2003.) |
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Theorem | setind2 4541 | Set (epsilon) induction, stated compactly. Given as a homework problem in 1992 by George Boolos (1940-1996). (Contributed by NM, 17-Sep-2003.) |
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Theorem | elirr 4542 |
No class is a member of itself. Exercise 6 of [TakeutiZaring] p. 22.
The reason that this theorem is marked as discouraged is a bit subtle.
If we wanted to reduce usage of ax-setind 4538, we could redefine
(Contributed by NM, 7-Aug-1994.) (Proof rewritten by Mario Carneiro and Jim Kingdon, 26-Nov-2018.) (New usage is discouraged.) |
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Theorem | ordirr 4543 | Epsilon irreflexivity of ordinals: no ordinal class is a member of itself. Theorem 2.2(i) of [BellMachover] p. 469, generalized to classes. The present proof requires ax-setind 4538. If in the definition of ordinals df-iord 4368, we also required that membership be well-founded on any ordinal (see df-frind 4334), then we could prove ordirr 4543 without ax-setind 4538. (Contributed by NM, 2-Jan-1994.) |
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Theorem | onirri 4544 | An ordinal number is not a member of itself. Theorem 7M(c) of [Enderton] p. 192. (Contributed by NM, 11-Jun-1994.) |
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Theorem | nordeq 4545 | A member of an ordinal class is not equal to it. (Contributed by NM, 25-May-1998.) |
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Theorem | ordn2lp 4546 | An ordinal class cannot be an element of one of its members. Variant of first part of Theorem 2.2(vii) of [BellMachover] p. 469. (Contributed by NM, 3-Apr-1994.) |
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Theorem | orddisj 4547 | An ordinal class and its singleton are disjoint. (Contributed by NM, 19-May-1998.) |
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Theorem | orddif 4548 | Ordinal derived from its successor. (Contributed by NM, 20-May-1998.) |
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Theorem | elirrv 4549 | The membership relation is irreflexive: no set is a member of itself. Theorem 105 of [Suppes] p. 54. (Contributed by NM, 19-Aug-1993.) |
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Theorem | sucprcreg 4550 | A class is equal to its successor iff it is a proper class (assuming the Axiom of Set Induction). (Contributed by NM, 9-Jul-2004.) |
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Theorem | ruv 4551 |
The Russell class is equal to the universe ![]() |
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Theorem | ruALT 4552 | Alternate proof of Russell's Paradox ru 2963, simplified using (indirectly) the Axiom of Set Induction ax-setind 4538. (Contributed by Alan Sare, 4-Oct-2008.) (Proof modification is discouraged.) (New usage is discouraged.) |
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Theorem | onprc 4553 | No set contains all ordinal numbers. Proposition 7.13 of [TakeutiZaring] p. 38. This is also known as the Burali-Forti paradox (remark in [Enderton] p. 194). In 1897, Cesare Burali-Forti noticed that since the "set" of all ordinal numbers is an ordinal class (ordon 4487), it must be both an element of the set of all ordinal numbers yet greater than every such element. ZF set theory resolves this paradox by not allowing the class of all ordinal numbers to be a set (so instead it is a proper class). Here we prove the denial of its existence. (Contributed by NM, 18-May-1994.) |
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Theorem | sucon 4554 | The class of all ordinal numbers is its own successor. (Contributed by NM, 12-Sep-2003.) |
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Theorem | en2lp 4555 | No class has 2-cycle membership loops. Theorem 7X(b) of [Enderton] p. 206. (Contributed by NM, 16-Oct-1996.) (Proof rewritten by Mario Carneiro and Jim Kingdon, 27-Nov-2018.) |
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Theorem | preleq 4556 | Equality of two unordered pairs when one member of each pair contains the other member. (Contributed by NM, 16-Oct-1996.) |
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Theorem | opthreg 4557 | Theorem for alternate representation of ordered pairs, requiring the Axiom of Set Induction ax-setind 4538 (via the preleq 4556 step). See df-op 3603 for a description of other ordered pair representations. Exercise 34 of [Enderton] p. 207. (Contributed by NM, 16-Oct-1996.) |
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Theorem | suc11g 4558 | The successor operation behaves like a one-to-one function (assuming the Axiom of Set Induction). Similar to Exercise 35 of [Enderton] p. 208 and its converse. (Contributed by NM, 25-Oct-2003.) |
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Theorem | suc11 4559 | The successor operation behaves like a one-to-one function. Compare Exercise 16 of [Enderton] p. 194. (Contributed by NM, 3-Sep-2003.) |
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Theorem | dtruex 4560* |
At least two sets exist (or in terms of first-order logic, the universe
of discourse has two or more objects). Although dtruarb 4193 can also be
summarized as "at least two sets exist", the difference is
that
dtruarb 4193 shows the existence of two sets which are not
equal to each
other, but this theorem says that given a specific ![]() ![]() |
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Theorem | dtru 4561* | At least two sets exist (or in terms of first-order logic, the universe of discourse has two or more objects). If we assumed the law of the excluded middle this would be equivalent to dtruex 4560. (Contributed by Jim Kingdon, 29-Dec-2018.) |
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Theorem | eunex 4562 | Existential uniqueness implies there is a value for which the wff argument is false. (Contributed by Jim Kingdon, 29-Dec-2018.) |
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Theorem | ordsoexmid 4563 | Weak linearity of ordinals implies the law of the excluded middle (that is, decidability of an arbitrary proposition). (Contributed by Mario Carneiro and Jim Kingdon, 29-Jan-2019.) |
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Theorem | ordsuc 4564 | The successor of an ordinal class is ordinal. (Contributed by NM, 3-Apr-1995.) (Constructive proof by Mario Carneiro and Jim Kingdon, 20-Jul-2019.) |
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Theorem | onsucuni2 4565 | A successor ordinal is the successor of its union. (Contributed by NM, 10-Dec-2004.) (Proof shortened by Andrew Salmon, 27-Aug-2011.) |
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Theorem | 0elsucexmid 4566* | If the successor of any ordinal class contains the empty set, excluded middle follows. (Contributed by Jim Kingdon, 3-Sep-2021.) |
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Theorem | nlimsucg 4567 | A successor is not a limit ordinal. (Contributed by NM, 25-Mar-1995.) (Proof shortened by Andrew Salmon, 27-Aug-2011.) |
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Theorem | ordpwsucss 4568 |
The collection of ordinals in the power class of an ordinal is a
superset of its successor.
We can think of
Constructively |
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Theorem | onnmin 4569 | No member of a set of ordinal numbers belongs to its minimum. (Contributed by NM, 2-Feb-1997.) (Constructive proof by Mario Carneiro and Jim Kingdon, 21-Jul-2019.) |
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Theorem | ssnel 4570 | Relationship between subset and elementhood. In the context of ordinals this can be seen as an ordering law. (Contributed by Jim Kingdon, 22-Jul-2019.) |
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Theorem | ordpwsucexmid 4571* | The subset in ordpwsucss 4568 cannot be equality. That is, strengthening it to equality implies excluded middle. (Contributed by Jim Kingdon, 30-Jul-2019.) |
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Theorem | ordtri2or2exmid 4572* | Ordinal trichotomy implies excluded middle. (Contributed by Jim Kingdon, 29-Aug-2021.) |
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Theorem | ontri2orexmidim 4573* | Ordinal trichotomy implies excluded middle. Closed form of ordtri2or2exmid 4572. (Contributed by Jim Kingdon, 26-Aug-2024.) |
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Theorem | onintexmid 4574* | If the intersection (infimum) of an inhabited class of ordinal numbers belongs to the class, excluded middle follows. The hypothesis would be provable given excluded middle. (Contributed by Mario Carneiro and Jim Kingdon, 29-Aug-2021.) |
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Theorem | zfregfr 4575 | The epsilon relation is well-founded on any class. (Contributed by NM, 26-Nov-1995.) |
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Theorem | ordfr 4576 | Epsilon is well-founded on an ordinal class. (Contributed by NM, 22-Apr-1994.) |
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Theorem | ordwe 4577 | Epsilon well-orders every ordinal. Proposition 7.4 of [TakeutiZaring] p. 36. (Contributed by NM, 3-Apr-1994.) |
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Theorem | wetriext 4578* | A trichotomous well-order is extensional. (Contributed by Jim Kingdon, 26-Sep-2021.) |
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Theorem | wessep 4579 | A subset of a set well-ordered by set membership is well-ordered by set membership. (Contributed by Jim Kingdon, 30-Sep-2021.) |
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Theorem | reg3exmidlemwe 4580* |
Lemma for reg3exmid 4581. Our counterexample ![]() ![]() |
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Theorem | reg3exmid 4581* |
If any inhabited set satisfying df-wetr 4336 for ![]() |
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Theorem | dcextest 4582* |
If it is decidable whether ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
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Theorem | tfi 4583* |
The Principle of Transfinite Induction. Theorem 7.17 of [TakeutiZaring]
p. 39. This principle states that if ![]() ![]() ![]() ![]() (Contributed by NM, 18-Feb-2004.) |
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Theorem | tfis 4584* |
Transfinite Induction Schema. If all ordinal numbers less than a given
number ![]() |
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Theorem | tfis2f 4585* | Transfinite Induction Schema, using implicit substitution. (Contributed by NM, 18-Aug-1994.) |
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Theorem | tfis2 4586* | Transfinite Induction Schema, using implicit substitution. (Contributed by NM, 18-Aug-1994.) |
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Theorem | tfis3 4587* | Transfinite Induction Schema, using implicit substitution. (Contributed by NM, 4-Nov-2003.) |
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Theorem | tfisi 4588* | A transfinite induction scheme in "implicit" form where the induction is done on an object derived from the object of interest. (Contributed by Stefan O'Rear, 24-Aug-2015.) |
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Axiom | ax-iinf 4589* | Axiom of Infinity. Axiom 5 of [Crosilla] p. "Axioms of CZF and IZF". (Contributed by Jim Kingdon, 16-Nov-2018.) |
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Theorem | zfinf2 4590* | A standard version of the Axiom of Infinity, using definitions to abbreviate. Axiom Inf of [BellMachover] p. 472. (Contributed by NM, 30-Aug-1993.) |
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Syntax | com 4591 | Extend class notation to include the class of natural numbers. |
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Definition | df-iom 4592* |
Define the class of natural numbers as the smallest inductive set, which
is valid provided we assume the Axiom of Infinity. Definition 6.3 of
[Eisenberg] p. 82.
Note: the natural numbers We are unable to use the terms finite ordinal and natural number interchangeably, as shown at exmidonfin 7195. (Contributed by NM, 6-Aug-1994.) Use its alias dfom3 4593 instead for naming consistency with set.mm. (New usage is discouraged.) |
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Theorem | dfom3 4593* | Alias for df-iom 4592. Use it instead of df-iom 4592 for naming consistency with set.mm. (Contributed by NM, 6-Aug-1994.) |
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Theorem | omex 4594 | The existence of omega (the class of natural numbers). Axiom 7 of [TakeutiZaring] p. 43. (Contributed by NM, 6-Aug-1994.) |
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Theorem | peano1 4595 | Zero is a natural number. One of Peano's five postulates for arithmetic. Proposition 7.30(1) of [TakeutiZaring] p. 42. (Contributed by NM, 15-May-1994.) |
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Theorem | peano2 4596 | The successor of any natural number is a natural number. One of Peano's five postulates for arithmetic. Proposition 7.30(2) of [TakeutiZaring] p. 42. (Contributed by NM, 3-Sep-2003.) |
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Theorem | peano3 4597 | The successor of any natural number is not zero. One of Peano's five postulates for arithmetic. Proposition 7.30(3) of [TakeutiZaring] p. 42. (Contributed by NM, 3-Sep-2003.) |
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Theorem | peano4 4598 | Two natural numbers are equal iff their successors are equal, i.e. the successor function is one-to-one. One of Peano's five postulates for arithmetic. Proposition 7.30(4) of [TakeutiZaring] p. 43. (Contributed by NM, 3-Sep-2003.) |
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Theorem | peano5 4599* | The induction postulate: any class containing zero and closed under the successor operation contains all natural numbers. One of Peano's five postulates for arithmetic. Proposition 7.30(5) of [TakeutiZaring] p. 43. The more traditional statement of mathematical induction as a theorem schema, with a basis and an induction step, is derived from this theorem as Theorem findes 4604. (Contributed by NM, 18-Feb-2004.) |
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Theorem | find 4600* |
The Principle of Finite Induction (mathematical induction). Corollary
7.31 of [TakeutiZaring] p. 43.
The simpler hypothesis shown here was
suggested in an email from "Colin" on 1-Oct-2001. The
hypothesis states
that ![]() ![]() ![]() ![]() ![]() |
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