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Theorem List for Intuitionistic Logic Explorer - 9201-9300   *Has distinct variable group(s)
TypeLabelDescription
Statement
 
Theoremabsval2 9201 Value of absolute value function. Definition 10.36 of [Gleason] p. 133. (Contributed by NM, 17-Mar-2005.)
(A ℂ → (abs‘A) = (√‘(((ℜ‘A)↑2) + ((ℑ‘A)↑2))))
 
Theoremabs0 9202 The absolute value of 0. (Contributed by NM, 26-Mar-2005.) (Revised by Mario Carneiro, 29-May-2016.)
(abs‘0) = 0
 
Theoremabsi 9203 The absolute value of the imaginary unit. (Contributed by NM, 26-Mar-2005.)
(abs‘i) = 1
 
Theoremabs00bd 9204 If a complex number is zero, its absolute value is zero. (Contributed by David Moews, 28-Feb-2017.)
(φA = 0)       (φ → (abs‘A) = 0)
 
Theoremabsid 9205 A nonnegative number is its own absolute value. (Contributed by NM, 11-Oct-1999.) (Revised by Mario Carneiro, 29-May-2016.)
((A 0 ≤ A) → (abs‘A) = A)
 
Theoremabs1 9206 The absolute value of 1. Common special case. (Contributed by David A. Wheeler, 16-Jul-2016.)
(abs‘1) = 1
 
Theoremabsnid 9207 A negative number is the negative of its own absolute value. (Contributed by NM, 27-Feb-2005.)
((A A ≤ 0) → (abs‘A) = -A)
 
Theoremabsre 9208 Absolute value of a real number. (Contributed by NM, 17-Mar-2005.)
(A ℝ → (abs‘A) = (√‘(A↑2)))
 
Theoremnn0abscl 9209 The absolute value of an integer is a nonnegative integer. (Contributed by NM, 27-Feb-2005.)
(A ℤ → (abs‘A) 0)
 
Theoremzabscl 9210 The absolute value of an integer is an integer. (Contributed by Stefan O'Rear, 24-Sep-2014.)
(A ℤ → (abs‘A) ℤ)
 
PART 4  GUIDES AND MISCELLANEA
 
4.1  Guides (conventions, explanations, and examples)
 
4.1.1  Conventions

This section describes the conventions we use. However, these conventions often refer to existing mathematical practices, which are discussed in more detail in other references. The following sources lay out how mathematics is developed without the law of the excluded middle. Of course, there are a greater number of sources which assume excluded middle and most of what is in them applies here too (especially in a treatment such as ours which is built on first order logic and set theory, rather than, say, type theory). Studying how a topic is treated in the Metamath Proof Explorer and the references therein is often a good place to start (and is easy to compare with the Intuitionistic Logic Explorer). The textbooks provide a motivation for what we are doing, whereas Metamath lets you see in detail all hidden and implicit steps. Most standard theorems are accompanied by citations. Some closely followed texts include the following:

  • Axioms of propositional calculus - Stanford Encyclopedia of Philosophy or [Heyting].
  • Axioms of predicate calculus - our axioms are adapted from the ones in the Metamath Proof Explorer.
  • Theorems of propositional calculus - [Heyting].
  • Theorems of pure predicate calculus - Metamath Proof Explorer.
  • Theorems of equality and substitution - Metamath Proof Explorer.
  • Axioms of set theory - [Crosilla].
  • Development of set theory - Chapter 10 of [HoTT].
  • Construction of real and complex numbers - Chapter 11 of [HoTT]; [BauerTaylor].
  • Theorems about real numbers - [Geuvers].
 
Theoremconventions 9211 Unless there is a reason to diverge, we follow the conventions of the Metamath Proof Explorer (aka "set.mm"). This list of conventions is intended to be read in conjunction with the corresponding conventions in the Metamath Proof Explorer, and only the differences are described below.

  • Minimizing axioms and the axiom of choice. We prefer proofs that depend on fewer and/or weaker axioms, even if the proofs are longer. In particular, our choice of IZF (Intuitionistic Zermelo-Fraenkel) over CZF (Constructive Zermelo-Fraenkel, a weaker system) was just an expedient choice because IZF is easier to formalize in metamath. You can find some development using CZF in BJ's mathbox starting at ax-bd0 9248 (and the section header just above it). As for the axiom of choice, the full axiom of choice implies excluded middle as seen at acexmid 5454, although some authors will use countable choice or dependent choice. For example, countable choice or excluded middle is needed to show that the Cauchy reals coincide with the Dedekind reals - Corollary 11.4.3 of [HoTT], p. (varies).
  • Junk/undefined results. Much of the discussion of this topic in the Metamath Proof Explorer applies except that certain techniques are not available to us. For example, the Metamath Proof Explorer will often say "if a function is evaluated within its domain, a certain result follows; if the function is evaluated outside its domain, the same result follows. Since the function must be evaluated within its domain or outside it, the result follows unconditionally" (the use of excluded middle in this argument is perhaps obvious when stated this way). For this reason, we generally need to prove we are evaluating functions within their domains and avoid the reverse closure theorems of the Metamath Proof Explorer.
  • Bibliography references. The bibliography for the Intuitionistic Logic Explorer is separate from the one for the Metamath Proof Explorer but feel free to copy-paste a citation in either direction in order to cite it.

Label naming conventions

Here are a few of the label naming conventions:

  • Suffixes. We follow the conventions of the Metamath Proof Explorer with a few additions. A biconditional in set.mm which is an implication in iset.mm should have a "r" (for the reverse direction), or "i"/"im" (for the forward direction) appended. A theorem in set.mm which has a decidability condition added should add "dc" to the theorem name. A theorem in set.mm where "nonempty class" is changed to "inhabited class" should add "m" (for member) to the theorem name.

The following table shows some commonly-used abbreviations in labels which are not found in the Metamath Proof Explorer, in alphabetical order. For each abbreviation we provide a mnenomic to help you remember it, the source theorem/assumption defining it, an expression showing what it looks like, whether or not it is a "syntax fragment" (an abbreviation that indicates a particular kind of syntax), and hyperlinks to label examples that use the abbreviation. The abbreviation is bolded if there is a df-NAME definition but the label fragment is not NAME.

AbbreviationMnenomicSource ExpressionSyntax?Example(s)
apapart df-ap 7346 Yes apadd1 7372, apne 7387

  • Community. The Metamath mailing list also covers the Intuitionistic Logic Explorer and is at: https://groups.google.com/forum/#!forum/metamath.
  • (Contributed by Jim Kingdon, 24-Feb-2020.)

    φ       φ
     
    PART 5  SUPPLEMENTARY MATERIAL (USER'S MATHBOXES)
     
    5.1  Mathboxes for user contributions
     
    5.1.1  Mathbox guidelines
     
    Theoremmathbox 9212 (This theorem is a dummy placeholder for these guidelines. The name of this theorem, "mathbox", is hard-coded into the Metamath program to identify the start of the mathbox section for web page generation.)

    A "mathbox" is a user-contributed section that is maintained by its contributor independently from the main part of set.mm.

    For contributors:

    By making a contribution, you agree to release it into the public domain, according to the statement at the beginning of set.mm.

    Mathboxes are provided to help keep your work synchronized with changes in set.mm, but they shouldn't be depended on as a permanent archive. If you want to preserve your original contribution, it is your responsibility to keep your own copy of it along with the version of set.mm that works with it.

    Guidelines:

    1. If at all possible, please use only 0-ary class constants for new definitions.

    2. Try to follow the style of the rest of set.mm. Each $p and $a statement must be immediately preceded with the comment that will be shown on its web page description. The metamath program command "write source set.mm /rewrap" will take care of wrapping comment lines and indentation conventions. All mathbox content will be on public display and should hopefully reflect the overall quality of the website.

    3. Before submitting a revised mathbox, please make sure it verifies against the current set.mm.

    4. Mathboxes should be independent i.e. the proofs should verify with all other mathboxes removed. If you need a theorem from another mathbox, that is fine (and encouraged), but let me know, so I can move the theorem to the main section. One way avoid undesired accidental use of other mathbox theorems is to develop your mathbox using a modified set.mm that has mathboxes removed.

    Notes:

    1. We may decide to move some theorems to the main part of set.mm for general use.

    2. We may make changes to mathboxes to maintain the overall quality of set.mm. Normally we will let you know if a change might impact what you are working on.

    3. If you use theorems from another user's mathbox, we don't provide assurance that they are based on correct or consistent $a statements. (If you find such a problem, please let us know so it can be corrected.) (Contributed by NM, 20-Feb-2007.) (New usage is discouraged.)

    x = x
     
    5.2  Mathbox for Mykola Mostovenko
     
    Theoremax1hfs 9213 Heyting's formal system Axiom #1 from [Heyting] p. 127. (Contributed by MM, 11-Aug-2018.)
    (φ → (φ φ))
     
    5.3  Mathbox for BJ
     
    5.3.1  Propositional calculus
     
    Theoremnnexmid 9214 Double negation of excluded middle. Intuitionistic logic refutes the negation of excluded middle (but, of course, does not prove excluded middle) for any formula. (Contributed by BJ, 9-Oct-2019.)
    ¬ ¬ (φ ¬ φ)
     
    Theoremnndc 9215 Double negation of decidability of a formula. Intuitionistic logic refutes undecidability (but, of course, does not prove decidability) of any formula. (Contributed by BJ, 9-Oct-2019.)
    ¬ ¬ DECID φ
     
    Theoremdcdc 9216 Decidability of a proposition is decidable if and only if that proposition is decidable. DECID is idempotent. (Contributed by BJ, 9-Oct-2019.)
    (DECID DECID φDECID φ)
     
    5.3.2  Predicate calculus
     
    Theorembj-ex 9217* Existential generalization. (Contributed by BJ, 8-Dec-2019.) Proof modification is discouraged because there are shorter proofs, but using less basic results (like exlimiv 1486 and 19.9ht 1529 or 19.23ht 1383). (Proof modification is discouraged.)
    (xφφ)
     
    Theorembj-hbalt 9218 Closed form of hbal 1363 (copied from set.mm). (Contributed by BJ, 2-May-2019.)
    (y(φxφ) → (yφxyφ))
     
    Theorembj-nfalt 9219 Closed form of nfal 1465 (copied from set.mm). (Contributed by BJ, 2-May-2019.)
    (xyφ → Ⅎyxφ)
     
    Theoremspimd 9220 Deduction form of spim 1623. (Contributed by BJ, 17-Oct-2019.)
    (φ → Ⅎxχ)    &   (φx(x = y → (ψχ)))       (φ → (xψχ))
     
    Theorem2spim 9221* Double substitution, as in spim 1623. (Contributed by BJ, 17-Oct-2019.)
    xχ    &   zχ    &   ((x = y z = 𝑡) → (ψχ))       (zxψχ)
     
    Theoremch2var 9222* Implicit substitution of y for x and 𝑡 for z into a theorem. (Contributed by BJ, 17-Oct-2019.)
    xψ    &   zψ    &   ((x = y z = 𝑡) → (φψ))    &   φ       ψ
     
    Theoremch2varv 9223* Version of ch2var 9222 with non-freeness hypotheses replaced by DV conditions. (Contributed by BJ, 17-Oct-2019.)
    ((x = y z = 𝑡) → (φψ))    &   φ       ψ
     
    Theorembj-exlimmp 9224 Lemma for bj-vtoclgf 9230. (Contributed by BJ, 21-Nov-2019.) (Proof modification is discouraged.)
    xψ    &   (χφ)       (x(χ → (φψ)) → (xχψ))
     
    Theorembj-exlimmpi 9225 Lemma for bj-vtoclgf 9230. (Contributed by BJ, 21-Nov-2019.) (Proof modification is discouraged.)
    xψ    &   (χφ)    &   (χ → (φψ))       (xχψ)
     
    Theorembj-sbimedh 9226 A strengthening of sbiedh 1667 (same proof). (Contributed by BJ, 16-Dec-2019.)
    (φxφ)    &   (φ → (χxχ))    &   (φ → (x = y → (ψχ)))       (φ → ([y / x]ψχ))
     
    Theorembj-sbimeh 9227 A strengthening of sbieh 1670 (same proof). (Contributed by BJ, 16-Dec-2019.)
    (ψxψ)    &   (x = y → (φψ))       ([y / x]φψ)
     
    Theorembj-sbime 9228 A strengthening of sbie 1671 (same proof). (Contributed by BJ, 16-Dec-2019.)
    xψ    &   (x = y → (φψ))       ([y / x]φψ)
     
    5.3.3  Extensionality

    Various utility theorems using FOL and extensionality.

     
    Theorembj-vtoclgft 9229 Weakening two hypotheses of vtoclgf 2606. (Contributed by BJ, 21-Nov-2019.)
    xA    &   xψ    &   (x = Aφ)       (x(x = A → (φψ)) → (A 𝑉ψ))
     
    Theorembj-vtoclgf 9230 Weakening two hypotheses of vtoclgf 2606. (Contributed by BJ, 21-Nov-2019.)
    xA    &   xψ    &   (x = Aφ)    &   (x = A → (φψ))       (A 𝑉ψ)
     
    Theoremelabgf0 9231 Lemma for elabgf 2679. (Contributed by BJ, 21-Nov-2019.)
    (x = A → (A {xφ} ↔ φ))
     
    Theoremelabgft1 9232 One implication of elabgf 2679, in closed form. (Contributed by BJ, 21-Nov-2019.)
    xA    &   xψ       (x(x = A → (φψ)) → (A {xφ} → ψ))
     
    Theoremelabgf1 9233 One implication of elabgf 2679. (Contributed by BJ, 21-Nov-2019.)
    xA    &   xψ    &   (x = A → (φψ))       (A {xφ} → ψ)
     
    Theoremelabgf2 9234 One implication of elabgf 2679. (Contributed by BJ, 21-Nov-2019.)
    xA    &   xψ    &   (x = A → (ψφ))       (A B → (ψA {xφ}))
     
    Theoremelabf1 9235* One implication of elabf 2680. (Contributed by BJ, 21-Nov-2019.)
    xψ    &   (x = A → (φψ))       (A {xφ} → ψ)
     
    Theoremelabf2 9236* One implication of elabf 2680. (Contributed by BJ, 21-Nov-2019.)
    xψ    &   A V    &   (x = A → (ψφ))       (ψA {xφ})
     
    Theoremelab1 9237* One implication of elab 2681. (Contributed by BJ, 21-Nov-2019.)
    (x = A → (φψ))       (A {xφ} → ψ)
     
    Theoremelab2a 9238* One implication of elab 2681. (Contributed by BJ, 21-Nov-2019.)
    A V    &   (x = A → (ψφ))       (ψA {xφ})
     
    Theoremelabg2 9239* One implication of elabg 2682. (Contributed by BJ, 21-Nov-2019.)
    (x = A → (ψφ))       (A 𝑉 → (ψA {xφ}))
     
    Theorembj-rspgt 9240 Restricted specialization, generalized. Weakens a hypothesis of rspccv 2647 and seems to have a shorter proof. (Contributed by BJ, 21-Nov-2019.)
    xA    &   xB    &   xψ       (x(x = A → (φψ)) → (x B φ → (A Bψ)))
     
    Theorembj-rspg 9241 Restricted specialization, generalized. Weakens a hypothesis of rspccv 2647 and seems to have a shorter proof. (Contributed by BJ, 21-Nov-2019.)
    xA    &   xB    &   xψ    &   (x = A → (φψ))       (x B φ → (A Bψ))
     
    Theoremcbvrald 9242* Rule used to change bound variables, using implicit substitution. (Contributed by BJ, 22-Nov-2019.)
    xφ    &   yφ    &   (φ → Ⅎyψ)    &   (φ → Ⅎxχ)    &   (φ → (x = y → (ψχ)))       (φ → (x A ψy A χ))
     
    Theorembj-intabssel 9243 Version of intss1 3621 using a class abstraction and explicit substitution. (Contributed by BJ, 29-Nov-2019.)
    xA       (A 𝑉 → ([A / x]φ {xφ} ⊆ A))
     
    Theorembj-intabssel1 9244 Version of intss1 3621 using a class abstraction and implicit substitution. Closed form of intmin3 3633. (Contributed by BJ, 29-Nov-2019.)
    xA    &   xψ    &   (x = A → (ψφ))       (A 𝑉 → (ψ {xφ} ⊆ A))
     
    Theorembj-elssuniab 9245 Version of elssuni 3599 using a class abstraction and explicit substitution. (Contributed by BJ, 29-Nov-2019.)
    xA       (A 𝑉 → ([A / x]φA {xφ}))
     
    Theorembj-sseq 9246 If two converse inclusions are characterized each by a formula, then equality is characterized by the conjunction of these formulas. (Contributed by BJ, 30-Nov-2019.)
    (φ → (ψAB))    &   (φ → (χBA))       (φ → ((ψ χ) ↔ A = B))
     
    5.3.4  Bounded formulas

    This is an ongoing project to define bounded formulas, following a discussion on GitHub between Jim Kingdon, Mario Carneiro and I, started 23-Sept-2019 (see https://github.com/metamath/set.mm/issues/1173 and links therein).

    In order to state certain axiom schemes of Constructive Zermelo–Fraenkel (CZF) set theory, like the axiom scheme of bounded (or restricted, or Δ0) separation, it is necessary to distinguish certain formulas, called bounded (or restricted, or Δ0) formulas. The necessity of considering bounded formulas also arises in several theories of bounded arithmetic, both classical or intuitonistic, for instance to state the axiom scheme of Δ0-induction.

    To formalize this in Metamath, there are several choices to make.

    A first choice is to either create a new type for bounded formulas, or to create a predicate on formulas that indicates whether they are bounded. In the first case, one creates a new type "wff0" with a new set of metavariables (ph0 ...) and an axiom "$a wff ph0 " ensuring that bounded formulas are formulas, so that one can reuse existing theorems, and then axioms take the form "$a wff0 ( ph0 -> ps0 )", etc. In the second case, one introduces a predicate "BOUNDED " with the intended meaning that "BOUNDED φ " is a formula meaning that φ is a bounded formula. We choose the second option, since the first would complicate the grammar, risking to make it ambiguous. (TODO: elaborate.)

    A second choice is to view "bounded" either as a syntactic or a semantic property. For instance, x is not syntactically bounded since it has an unbounded universal quantifier, but it is semantically bounded since it is equivalent to which is bounded. We choose the second option, so that formulas using defined symbols can be proved bounded.

    A third choice is in the form of the axioms, either in closed form or in inference form. One cannot state all the axioms in closed form, especially ax-bd0 9248. Indeed, if we posited it in closed form, then we could prove for instance (φBOUNDED φ) and φBOUNDED φ) which is problematic (with the law of excluded middle, this would entail that all formulas are bounded, but even without it, too many formulas could be proved bounded...). (TODO: elaborate.)

    Having ax-bd0 9248 in inference form ensures that a formula can be proved bounded only if it is equivalent *for all values of the free variables* to a syntactically bounded one. The other axioms (ax-bdim 9249 through ax-bdsb 9257) can be written either in closed or inference form. The fact that ax-bd0 9248 is an inference is enough to ensure that the closed forms cannot be "exploited" to prove that some unbounded formulas are bounded. (TODO: check.) However, we state all the axioms in inference form to make it clear that we do not exploit any over-permissiveness.

    Finally, note that our logic has no terms, only variables. Therefore, we cannot prove for instance that x 𝜔 is a bounded formula. However, since 𝜔 can be defined as "the y such that PHI" a proof using the fact that x 𝜔 is bounded can be converted to a proof in iset.mm by replacing 𝜔 with y everywhere and prepending the antecedent PHI, since x y is bounded by ax-bdel 9256. For a similar method, see bj-omtrans 9390.

    Note that one cannot add an axiom BOUNDED x A since by bdph 9285 it would imply that every formula is bounded.

    For CZF, a useful set of notes is

    Peter Aczel and Michael Rathjen, CST Book draft. (available at http://www1.maths.leeds.ac.uk/~rathjen/book.pdf)

    and an interesting article is

    Michael Shulman, Comparing material and structural set theories, Annals of Pure and Applied Logic, Volume 170, Issue 4 (Apr. 2019), 465--504. (available at https://arxiv.org/abs/1808.05204)

     
    Syntaxwbd 9247 Syntax for the predicate BOUNDED.
    wff BOUNDED φ
     
    Axiomax-bd0 9248 If two formulas are equivalent, then boundedness of one implies boundedness of the other. (Contributed by BJ, 3-Oct-2019.)
    (φψ)       (BOUNDED φBOUNDED ψ)
     
    Axiomax-bdim 9249 An implication between two bounded formulas is bounded. (Contributed by BJ, 25-Sep-2019.)
    BOUNDED φ    &   BOUNDED ψ       BOUNDED (φψ)
     
    Axiomax-bdan 9250 The conjunction of two bounded formulas is bounded. (Contributed by BJ, 25-Sep-2019.)
    BOUNDED φ    &   BOUNDED ψ       BOUNDED (φ ψ)
     
    Axiomax-bdor 9251 The disjunction of two bounded formulas is bounded. (Contributed by BJ, 25-Sep-2019.)
    BOUNDED φ    &   BOUNDED ψ       BOUNDED (φ ψ)
     
    Axiomax-bdn 9252 The negation of a bounded formula is bounded. (Contributed by BJ, 25-Sep-2019.)
    BOUNDED φ       BOUNDED ¬ φ
     
    Axiomax-bdal 9253* A bounded universal quantification of a bounded formula is bounded. Note the DV condition on x, y. (Contributed by BJ, 25-Sep-2019.)
    BOUNDED φ       BOUNDED x y φ
     
    Axiomax-bdex 9254* A bounded existential quantification of a bounded formula is bounded. Note the DV condition on x, y. (Contributed by BJ, 25-Sep-2019.)
    BOUNDED φ       BOUNDED x y φ
     
    Axiomax-bdeq 9255 An atomic formula is bounded (equality predicate). (Contributed by BJ, 3-Oct-2019.)
    BOUNDED x = y
     
    Axiomax-bdel 9256 An atomic formula is bounded (membership predicate). (Contributed by BJ, 3-Oct-2019.)
    BOUNDED x y
     
    Axiomax-bdsb 9257 A formula resulting from proper substitution in a bounded formula is bounded. This probably cannot be proved from the other axioms, since neither the definiens in df-sb 1643, nor probably any other equivalent formula, is syntactically bounded. (Contributed by BJ, 3-Oct-2019.)
    BOUNDED φ       BOUNDED [y / x]φ
     
    Theorembdeq 9258 Equality property for the predicate BOUNDED. (Contributed by BJ, 3-Oct-2019.)
    (φψ)       (BOUNDED φBOUNDED ψ)
     
    Theorembd0 9259 A formula equivalent to a bounded one is bounded. See also bd0r 9260. (Contributed by BJ, 3-Oct-2019.)
    BOUNDED φ    &   (φψ)       BOUNDED ψ
     
    Theorembd0r 9260 A formula equivalent to a bounded one is bounded. Stated with a commuted (compared with bd0 9259) biconditional in the hypothesis, to work better with definitions (ψ is the definiendum that one wants to prove bounded). (Contributed by BJ, 3-Oct-2019.)
    BOUNDED φ    &   (ψφ)       BOUNDED ψ
     
    Theorembdbi 9261 A biconditional between two bounded formulas is bounded. (Contributed by BJ, 3-Oct-2019.)
    BOUNDED φ    &   BOUNDED ψ       BOUNDED (φψ)
     
    Theorembdstab 9262 Stability of a bounded formula is bounded. (Contributed by BJ, 3-Oct-2019.)
    BOUNDED φ       BOUNDED STAB φ
     
    Theorembddc 9263 Decidability of a bounded formula is bounded. (Contributed by BJ, 3-Oct-2019.)
    BOUNDED φ       BOUNDED DECID φ
     
    Theorembd3or 9264 A disjunction of three bounded formulas is bounded. (Contributed by BJ, 3-Oct-2019.)
    BOUNDED φ    &   BOUNDED ψ    &   BOUNDED χ       BOUNDED (φ ψ χ)
     
    Theorembd3an 9265 A conjunction of three bounded formulas is bounded. (Contributed by BJ, 3-Oct-2019.)
    BOUNDED φ    &   BOUNDED ψ    &   BOUNDED χ       BOUNDED (φ ψ χ)
     
    Theorembdth 9266 A truth (a (closed) theorem) is a bounded formula. (Contributed by BJ, 6-Oct-2019.)
    φ       BOUNDED φ
     
    Theorembdtru 9267 The truth value is bounded. (Contributed by BJ, 3-Oct-2019.)
    BOUNDED
     
    Theorembdfal 9268 The truth value is bounded. (Contributed by BJ, 3-Oct-2019.)
    BOUNDED
     
    Theorembdnth 9269 A falsity is a bounded formula. (Contributed by BJ, 6-Oct-2019.)
    ¬ φ       BOUNDED φ
     
    TheorembdnthALT 9270 Alternate proof of bdnth 9269 not using bdfal 9268. Then, bdfal 9268 can be proved from this theorem, using fal 1249. The total number of proof steps would be 17 (for bdnthALT 9270) + 3 = 20, which is more than 8 (for bdfal 9268) + 9 (for bdnth 9269) = 17. (Contributed by BJ, 6-Oct-2019.) (Proof modification is discouraged.) (New usage is discouraged.)
    ¬ φ       BOUNDED φ
     
    Theorembdxor 9271 The exclusive disjunction of two bounded formulas is bounded. (Contributed by BJ, 3-Oct-2019.)
    BOUNDED φ    &   BOUNDED ψ       BOUNDED (φψ)
     
    Theorembj-bdcel 9272* Boundedness of a membership formula. (Contributed by BJ, 8-Dec-2019.)
    BOUNDED y = A       BOUNDED A x
     
    Theorembdab 9273 Membership in a class defined by class abstraction using a bounded formula, is a bounded formula. (Contributed by BJ, 3-Oct-2019.)
    BOUNDED φ       BOUNDED x {yφ}
     
    Theorembdcdeq 9274 Conditional equality of a bounded formula is a bounded formula. (Contributed by BJ, 16-Oct-2019.)
    BOUNDED φ       BOUNDED CondEq(x = yφ)
     
    5.3.5  Bounded classes

    In line with our definitions of classes as extensions of predicates, it is useful to define a predicate for bounded classes, which is done in df-bdc 9276. Note that this notion is only a technical device which can be used to shorten proofs of (semantic) boundedness of formulas.

    As will be clear by the end of this subsection (see for instance bdop 9310), one can prove the boundedness of any concrete term using only setvars and bounded formulas, for instance, BOUNDED φ BOUNDED ⟨{xφ}, ({y, suc z} × ⟨𝑡, ∅⟩)⟩. The proofs are long since one has to prove boundedness at each step of the construction, without being able to prove general theorems like BOUNDED ABOUNDED {A}.

     
    Syntaxwbdc 9275 Syntax for the predicate BOUNDED.
    wff BOUNDED A
     
    Definitiondf-bdc 9276* Define a bounded class as one such that membership in this class is a bounded formula. (Contributed by BJ, 3-Oct-2019.)
    (BOUNDED AxBOUNDED x A)
     
    Theorembdceq 9277 Equality property for the predicate BOUNDED. (Contributed by BJ, 3-Oct-2019.)
    A = B       (BOUNDED ABOUNDED B)
     
    Theorembdceqi 9278 A class equal to a bounded one is bounded. Note the use of ax-ext 2019. See also bdceqir 9279. (Contributed by BJ, 3-Oct-2019.)
    BOUNDED A    &   A = B       BOUNDED B
     
    Theorembdceqir 9279 A class equal to a bounded one is bounded. Stated with a commuted (compared with bdceqi 9278) equality in the hypothesis, to work better with definitions (B is the definiendum that one wants to prove bounded; see comment of bd0r 9260). (Contributed by BJ, 3-Oct-2019.)
    BOUNDED A    &   B = A       BOUNDED B
     
    Theorembdel 9280* The belonging of a setvar in a bounded class is a bounded formula. (Contributed by BJ, 3-Oct-2019.)
    (BOUNDED ABOUNDED x A)
     
    Theorembdeli 9281* Inference associated with bdel 9280. Its converse is bdelir 9282. (Contributed by BJ, 3-Oct-2019.)
    BOUNDED A       BOUNDED x A
     
    Theorembdelir 9282* Inference associated with df-bdc 9276. Its converse is bdeli 9281. (Contributed by BJ, 3-Oct-2019.)
    BOUNDED x A       BOUNDED A
     
    Theorembdcv 9283 A setvar is a bounded class. (Contributed by BJ, 3-Oct-2019.)
    BOUNDED x
     
    Theorembdcab 9284 A class defined by class abstraction using a bounded formula is bounded. (Contributed by BJ, 6-Oct-2019.)
    BOUNDED φ       BOUNDED {xφ}
     
    Theorembdph 9285 A formula which defines (by class abstraction) a bounded class is bounded. (Contributed by BJ, 6-Oct-2019.)
    BOUNDED {xφ}       BOUNDED φ
     
    Theorembds 9286* Boundedness of a formula resulting from implicit substitution in a bounded formula. Note that the proof does not use ax-bdsb 9257; therefore, using implicit instead of explicit substitution when boundedness is important, one might avoid using ax-bdsb 9257. (Contributed by BJ, 19-Nov-2019.)
    BOUNDED φ    &   (x = y → (φψ))       BOUNDED ψ
     
    Theorembdcrab 9287* A class defined by restricted abstraction from a bounded class and a bounded formula is bounded. (Contributed by BJ, 3-Oct-2019.)
    BOUNDED A    &   BOUNDED φ       BOUNDED {x Aφ}
     
    Theorembdne 9288 Inequality of two setvars is a bounded formula. (Contributed by BJ, 16-Oct-2019.)
    BOUNDED xy
     
    Theorembdnel 9289* Non-membership of a setvar in a bounded formula is a bounded formula. (Contributed by BJ, 16-Oct-2019.)
    BOUNDED A       BOUNDED xA
     
    Theorembdreu 9290* Boundedness of existential uniqueness.

    Remark regarding restricted quantifiers: the formula x Aφ need not be bounded even if A and φ are. Indeed, V is bounded by bdcvv 9292, and (x Vφxφ) (in minimal propositional calculus), so by bd0 9259, if x Vφ were bounded when φ is bounded, then xφ would be bounded as well when φ is bounded, which is not the case. The same remark holds with , ∃!, ∃*. (Contributed by BJ, 16-Oct-2019.)

    BOUNDED φ       BOUNDED ∃!x y φ
     
    Theorembdrmo 9291* Boundedness of existential at-most-one. (Contributed by BJ, 16-Oct-2019.)
    BOUNDED φ       BOUNDED ∃*x y φ
     
    Theorembdcvv 9292 The universal class is bounded. The formulation may sound strange, but recall that here, "bounded" means "Δ0". (Contributed by BJ, 3-Oct-2019.)
    BOUNDED V
     
    Theorembdsbc 9293 A formula resulting from proper substitution of a setvar for a setvar in a bounded formula is bounded. See also bdsbcALT 9294. (Contributed by BJ, 16-Oct-2019.)
    BOUNDED φ       BOUNDED [y / x]φ
     
    TheorembdsbcALT 9294 Alternate proof of bdsbc 9293. (Contributed by BJ, 16-Oct-2019.) (Proof modification is discouraged.) (New usage is discouraged.)
    BOUNDED φ       BOUNDED [y / x]φ
     
    Theorembdccsb 9295 A class resulting from proper substitution of a setvar for a setvar in a bounded class is bounded. (Contributed by BJ, 16-Oct-2019.)
    BOUNDED A       BOUNDED y / xA
     
    Theorembdcdif 9296 The difference of two bounded classes is bounded. (Contributed by BJ, 3-Oct-2019.)
    BOUNDED A    &   BOUNDED B       BOUNDED (AB)
     
    Theorembdcun 9297 The union of two bounded classes is bounded. (Contributed by BJ, 3-Oct-2019.)
    BOUNDED A    &   BOUNDED B       BOUNDED (AB)
     
    Theorembdcin 9298 The intersection of two bounded classes is bounded. (Contributed by BJ, 3-Oct-2019.)
    BOUNDED A    &   BOUNDED B       BOUNDED (AB)
     
    Theorembdss 9299 The inclusion of a setvar in a bounded class is a bounded formula. Note: apparently, we cannot prove from the present axioms that equality of two bounded classes is a bounded formula. (Contributed by BJ, 3-Oct-2019.)
    BOUNDED A       BOUNDED xA
     
    Theorembdcnul 9300 The empty class is bounded. See also bdcnulALT 9301. (Contributed by BJ, 3-Oct-2019.)
    BOUNDED
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