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Theorem List for Intuitionistic Logic Explorer - 6101-6200   *Has distinct variable group(s)
TypeLabelDescription
Statement

Theoremnnawordex 6101* Equivalence for weak ordering of natural numbers. (Contributed by NM, 8-Nov-2002.) (Revised by Mario Carneiro, 15-Nov-2014.)
((𝐴 ∈ ω ∧ 𝐵 ∈ ω) → (𝐴𝐵 ↔ ∃𝑥 ∈ ω (𝐴 +𝑜 𝑥) = 𝐵))

Theoremnnm00 6102 The product of two natural numbers is zero iff at least one of them is zero. (Contributed by Jim Kingdon, 11-Nov-2004.)
((𝐴 ∈ ω ∧ 𝐵 ∈ ω) → ((𝐴 ·𝑜 𝐵) = ∅ ↔ (𝐴 = ∅ ∨ 𝐵 = ∅)))

2.6.24  Equivalence relations and classes

Syntaxwer 6103 Extend the definition of a wff to include the equivalence predicate.
wff 𝑅 Er 𝐴

Syntaxcec 6104 Extend the definition of a class to include equivalence class.
class [𝐴]𝑅

Syntaxcqs 6105 Extend the definition of a class to include quotient set.
class (𝐴 / 𝑅)

Definitiondf-er 6106 Define the equivalence relation predicate. Our notation is not standard. A formal notation doesn't seem to exist in the literature; instead only informal English tends to be used. The present definition, although somewhat cryptic, nicely avoids dummy variables. In dfer2 6107 we derive a more typical definition. We show that an equivalence relation is reflexive, symmetric, and transitive in erref 6126, ersymb 6120, and ertr 6121. (Contributed by NM, 4-Jun-1995.) (Revised by Mario Carneiro, 2-Nov-2015.)
(𝑅 Er 𝐴 ↔ (Rel 𝑅 ∧ dom 𝑅 = 𝐴 ∧ (𝑅 ∪ (𝑅𝑅)) ⊆ 𝑅))

Theoremdfer2 6107* Alternate definition of equivalence predicate. (Contributed by NM, 3-Jan-1997.) (Revised by Mario Carneiro, 12-Aug-2015.)
(𝑅 Er 𝐴 ↔ (Rel 𝑅 ∧ dom 𝑅 = 𝐴 ∧ ∀𝑥𝑦𝑧((𝑥𝑅𝑦𝑦𝑅𝑥) ∧ ((𝑥𝑅𝑦𝑦𝑅𝑧) → 𝑥𝑅𝑧))))

Definitiondf-ec 6108 Define the 𝑅-coset of 𝐴. Exercise 35 of [Enderton] p. 61. This is called the equivalence class of 𝐴 modulo 𝑅 when 𝑅 is an equivalence relation (i.e. when Er 𝑅; see dfer2 6107). In this case, 𝐴 is a representative (member) of the equivalence class [𝐴]𝑅, which contains all sets that are equivalent to 𝐴. Definition of [Enderton] p. 57 uses the notation [𝐴] (subscript) 𝑅, although we simply follow the brackets by 𝑅 since we don't have subscripted expressions. For an alternate definition, see dfec2 6109. (Contributed by NM, 23-Jul-1995.)
[𝐴]𝑅 = (𝑅 “ {𝐴})

Theoremdfec2 6109* Alternate definition of 𝑅-coset of 𝐴. Definition 34 of [Suppes] p. 81. (Contributed by NM, 3-Jan-1997.) (Proof shortened by Mario Carneiro, 9-Jul-2014.)
(𝐴𝑉 → [𝐴]𝑅 = {𝑦𝐴𝑅𝑦})

Theoremecexg 6110 An equivalence class modulo a set is a set. (Contributed by NM, 24-Jul-1995.)
(𝑅𝐵 → [𝐴]𝑅 ∈ V)

Theoremecexr 6111 An inhabited equivalence class implies the representative is a set. (Contributed by Mario Carneiro, 9-Jul-2014.)
(𝐴 ∈ [𝐵]𝑅𝐵 ∈ V)

Definitiondf-qs 6112* Define quotient set. 𝑅 is usually an equivalence relation. Definition of [Enderton] p. 58. (Contributed by NM, 23-Jul-1995.)
(𝐴 / 𝑅) = {𝑦 ∣ ∃𝑥𝐴 𝑦 = [𝑥]𝑅}

Theoremereq1 6113 Equality theorem for equivalence predicate. (Contributed by NM, 4-Jun-1995.) (Revised by Mario Carneiro, 12-Aug-2015.)
(𝑅 = 𝑆 → (𝑅 Er 𝐴𝑆 Er 𝐴))

Theoremereq2 6114 Equality theorem for equivalence predicate. (Contributed by Mario Carneiro, 12-Aug-2015.)
(𝐴 = 𝐵 → (𝑅 Er 𝐴𝑅 Er 𝐵))

Theoremerrel 6115 An equivalence relation is a relation. (Contributed by Mario Carneiro, 12-Aug-2015.)
(𝑅 Er 𝐴 → Rel 𝑅)

Theoremerdm 6116 The domain of an equivalence relation. (Contributed by Mario Carneiro, 12-Aug-2015.)
(𝑅 Er 𝐴 → dom 𝑅 = 𝐴)

Theoremercl 6117 Elementhood in the field of an equivalence relation. (Contributed by Mario Carneiro, 12-Aug-2015.)
(𝜑𝑅 Er 𝑋)    &   (𝜑𝐴𝑅𝐵)       (𝜑𝐴𝑋)

Theoremersym 6118 An equivalence relation is symmetric. (Contributed by NM, 4-Jun-1995.) (Revised by Mario Carneiro, 12-Aug-2015.)
(𝜑𝑅 Er 𝑋)    &   (𝜑𝐴𝑅𝐵)       (𝜑𝐵𝑅𝐴)

Theoremercl2 6119 Elementhood in the field of an equivalence relation. (Contributed by Mario Carneiro, 12-Aug-2015.)
(𝜑𝑅 Er 𝑋)    &   (𝜑𝐴𝑅𝐵)       (𝜑𝐵𝑋)

Theoremersymb 6120 An equivalence relation is symmetric. (Contributed by NM, 30-Jul-1995.) (Revised by Mario Carneiro, 12-Aug-2015.)
(𝜑𝑅 Er 𝑋)       (𝜑 → (𝐴𝑅𝐵𝐵𝑅𝐴))

Theoremertr 6121 An equivalence relation is transitive. (Contributed by NM, 4-Jun-1995.) (Revised by Mario Carneiro, 12-Aug-2015.)
(𝜑𝑅 Er 𝑋)       (𝜑 → ((𝐴𝑅𝐵𝐵𝑅𝐶) → 𝐴𝑅𝐶))

Theoremertrd 6122 A transitivity relation for equivalences. (Contributed by Mario Carneiro, 9-Jul-2014.)
(𝜑𝑅 Er 𝑋)    &   (𝜑𝐴𝑅𝐵)    &   (𝜑𝐵𝑅𝐶)       (𝜑𝐴𝑅𝐶)

Theoremertr2d 6123 A transitivity relation for equivalences. (Contributed by Mario Carneiro, 9-Jul-2014.)
(𝜑𝑅 Er 𝑋)    &   (𝜑𝐴𝑅𝐵)    &   (𝜑𝐵𝑅𝐶)       (𝜑𝐶𝑅𝐴)

Theoremertr3d 6124 A transitivity relation for equivalences. (Contributed by Mario Carneiro, 9-Jul-2014.)
(𝜑𝑅 Er 𝑋)    &   (𝜑𝐵𝑅𝐴)    &   (𝜑𝐵𝑅𝐶)       (𝜑𝐴𝑅𝐶)

Theoremertr4d 6125 A transitivity relation for equivalences. (Contributed by Mario Carneiro, 9-Jul-2014.)
(𝜑𝑅 Er 𝑋)    &   (𝜑𝐴𝑅𝐵)    &   (𝜑𝐶𝑅𝐵)       (𝜑𝐴𝑅𝐶)

Theoremerref 6126 An equivalence relation is reflexive on its field. Compare Theorem 3M of [Enderton] p. 56. (Contributed by Mario Carneiro, 6-May-2013.) (Revised by Mario Carneiro, 12-Aug-2015.)
(𝜑𝑅 Er 𝑋)    &   (𝜑𝐴𝑋)       (𝜑𝐴𝑅𝐴)

Theoremercnv 6127 The converse of an equivalence relation is itself. (Contributed by Mario Carneiro, 12-Aug-2015.)
(𝑅 Er 𝐴𝑅 = 𝑅)

Theoremerrn 6128 The range and domain of an equivalence relation are equal. (Contributed by Rodolfo Medina, 11-Oct-2010.) (Revised by Mario Carneiro, 12-Aug-2015.)
(𝑅 Er 𝐴 → ran 𝑅 = 𝐴)

Theoremerssxp 6129 An equivalence relation is a subset of the cartesian product of the field. (Contributed by Mario Carneiro, 12-Aug-2015.)
(𝑅 Er 𝐴𝑅 ⊆ (𝐴 × 𝐴))

Theoremerex 6130 An equivalence relation is a set if its domain is a set. (Contributed by Rodolfo Medina, 15-Oct-2010.) (Proof shortened by Mario Carneiro, 12-Aug-2015.)
(𝑅 Er 𝐴 → (𝐴𝑉𝑅 ∈ V))

Theoremerexb 6131 An equivalence relation is a set if and only if its domain is a set. (Contributed by Rodolfo Medina, 15-Oct-2010.) (Revised by Mario Carneiro, 12-Aug-2015.)
(𝑅 Er 𝐴 → (𝑅 ∈ V ↔ 𝐴 ∈ V))

Theoremiserd 6132* A reflexive, symmetric, transitive relation is an equivalence relation on its domain. (Contributed by Mario Carneiro, 9-Jul-2014.) (Revised by Mario Carneiro, 12-Aug-2015.)
(𝜑 → Rel 𝑅)    &   ((𝜑𝑥𝑅𝑦) → 𝑦𝑅𝑥)    &   ((𝜑 ∧ (𝑥𝑅𝑦𝑦𝑅𝑧)) → 𝑥𝑅𝑧)    &   (𝜑 → (𝑥𝐴𝑥𝑅𝑥))       (𝜑𝑅 Er 𝐴)

Theorembrdifun 6133 Evaluate the incomparability relation. (Contributed by Mario Carneiro, 9-Jul-2014.)
𝑅 = ((𝑋 × 𝑋) ∖ ( < < ))       ((𝐴𝑋𝐵𝑋) → (𝐴𝑅𝐵 ↔ ¬ (𝐴 < 𝐵𝐵 < 𝐴)))

Theoremswoer 6134* Incomparability under a strict weak partial order is an equivalence relation. (Contributed by Mario Carneiro, 9-Jul-2014.) (Revised by Mario Carneiro, 12-Aug-2015.)
𝑅 = ((𝑋 × 𝑋) ∖ ( < < ))    &   ((𝜑 ∧ (𝑦𝑋𝑧𝑋)) → (𝑦 < 𝑧 → ¬ 𝑧 < 𝑦))    &   ((𝜑 ∧ (𝑥𝑋𝑦𝑋𝑧𝑋)) → (𝑥 < 𝑦 → (𝑥 < 𝑧𝑧 < 𝑦)))       (𝜑𝑅 Er 𝑋)

Theoremswoord1 6135* The incomparability equivalence relation is compatible with the original order. (Contributed by Mario Carneiro, 31-Dec-2014.)
𝑅 = ((𝑋 × 𝑋) ∖ ( < < ))    &   ((𝜑 ∧ (𝑦𝑋𝑧𝑋)) → (𝑦 < 𝑧 → ¬ 𝑧 < 𝑦))    &   ((𝜑 ∧ (𝑥𝑋𝑦𝑋𝑧𝑋)) → (𝑥 < 𝑦 → (𝑥 < 𝑧𝑧 < 𝑦)))    &   (𝜑𝐵𝑋)    &   (𝜑𝐶𝑋)    &   (𝜑𝐴𝑅𝐵)       (𝜑 → (𝐴 < 𝐶𝐵 < 𝐶))

Theoremswoord2 6136* The incomparability equivalence relation is compatible with the original order. (Contributed by Mario Carneiro, 31-Dec-2014.)
𝑅 = ((𝑋 × 𝑋) ∖ ( < < ))    &   ((𝜑 ∧ (𝑦𝑋𝑧𝑋)) → (𝑦 < 𝑧 → ¬ 𝑧 < 𝑦))    &   ((𝜑 ∧ (𝑥𝑋𝑦𝑋𝑧𝑋)) → (𝑥 < 𝑦 → (𝑥 < 𝑧𝑧 < 𝑦)))    &   (𝜑𝐵𝑋)    &   (𝜑𝐶𝑋)    &   (𝜑𝐴𝑅𝐵)       (𝜑 → (𝐶 < 𝐴𝐶 < 𝐵))

Theoremeqerlem 6137* Lemma for eqer 6138. (Contributed by NM, 17-Mar-2008.) (Proof shortened by Mario Carneiro, 6-Dec-2016.)
(𝑥 = 𝑦𝐴 = 𝐵)    &   𝑅 = {⟨𝑥, 𝑦⟩ ∣ 𝐴 = 𝐵}       (𝑧𝑅𝑤𝑧 / 𝑥𝐴 = 𝑤 / 𝑥𝐴)

Theoremeqer 6138* Equivalence relation involving equality of dependent classes 𝐴(𝑥) and 𝐵(𝑦). (Contributed by NM, 17-Mar-2008.) (Revised by Mario Carneiro, 12-Aug-2015.)
(𝑥 = 𝑦𝐴 = 𝐵)    &   𝑅 = {⟨𝑥, 𝑦⟩ ∣ 𝐴 = 𝐵}       𝑅 Er V

Theoremider 6139 The identity relation is an equivalence relation. (Contributed by NM, 10-May-1998.) (Proof shortened by Andrew Salmon, 22-Oct-2011.) (Proof shortened by Mario Carneiro, 9-Jul-2014.)
I Er V

Theorem0er 6140 The empty set is an equivalence relation on the empty set. (Contributed by Mario Carneiro, 5-Sep-2015.)
∅ Er ∅

Theoremeceq1 6141 Equality theorem for equivalence class. (Contributed by NM, 23-Jul-1995.)
(𝐴 = 𝐵 → [𝐴]𝐶 = [𝐵]𝐶)

Theoremeceq1d 6142 Equality theorem for equivalence class (deduction form). (Contributed by Jim Kingdon, 31-Dec-2019.)
(𝜑𝐴 = 𝐵)       (𝜑 → [𝐴]𝐶 = [𝐵]𝐶)

Theoremeceq2 6143 Equality theorem for equivalence class. (Contributed by NM, 23-Jul-1995.)
(𝐴 = 𝐵 → [𝐶]𝐴 = [𝐶]𝐵)

Theoremelecg 6144 Membership in an equivalence class. Theorem 72 of [Suppes] p. 82. (Contributed by Mario Carneiro, 9-Jul-2014.)
((𝐴𝑉𝐵𝑊) → (𝐴 ∈ [𝐵]𝑅𝐵𝑅𝐴))

Theoremelec 6145 Membership in an equivalence class. Theorem 72 of [Suppes] p. 82. (Contributed by NM, 23-Jul-1995.)
𝐴 ∈ V    &   𝐵 ∈ V       (𝐴 ∈ [𝐵]𝑅𝐵𝑅𝐴)

Theoremrelelec 6146 Membership in an equivalence class when 𝑅 is a relation. (Contributed by Mario Carneiro, 11-Sep-2015.)
(Rel 𝑅 → (𝐴 ∈ [𝐵]𝑅𝐵𝑅𝐴))

Theoremecss 6147 An equivalence class is a subset of the domain. (Contributed by NM, 6-Aug-1995.) (Revised by Mario Carneiro, 12-Aug-2015.)
(𝜑𝑅 Er 𝑋)       (𝜑 → [𝐴]𝑅𝑋)

Theoremecdmn0m 6148* A representative of an inhabited equivalence class belongs to the domain of the equivalence relation. (Contributed by Jim Kingdon, 21-Aug-2019.)
(𝐴 ∈ dom 𝑅 ↔ ∃𝑥 𝑥 ∈ [𝐴]𝑅)

Theoremereldm 6149 Equality of equivalence classes implies equivalence of domain membership. (Contributed by NM, 28-Jan-1996.) (Revised by Mario Carneiro, 12-Aug-2015.)
(𝜑𝑅 Er 𝑋)    &   (𝜑 → [𝐴]𝑅 = [𝐵]𝑅)       (𝜑 → (𝐴𝑋𝐵𝑋))

Theoremerth 6150 Basic property of equivalence relations. Theorem 73 of [Suppes] p. 82. (Contributed by NM, 23-Jul-1995.) (Revised by Mario Carneiro, 6-Jul-2015.)
(𝜑𝑅 Er 𝑋)    &   (𝜑𝐴𝑋)       (𝜑 → (𝐴𝑅𝐵 ↔ [𝐴]𝑅 = [𝐵]𝑅))

Theoremerth2 6151 Basic property of equivalence relations. Compare Theorem 73 of [Suppes] p. 82. Assumes membership of the second argument in the domain. (Contributed by NM, 30-Jul-1995.) (Revised by Mario Carneiro, 6-Jul-2015.)
(𝜑𝑅 Er 𝑋)    &   (𝜑𝐵𝑋)       (𝜑 → (𝐴𝑅𝐵 ↔ [𝐴]𝑅 = [𝐵]𝑅))

Theoremerthi 6152 Basic property of equivalence relations. Part of Lemma 3N of [Enderton] p. 57. (Contributed by NM, 30-Jul-1995.) (Revised by Mario Carneiro, 9-Jul-2014.)
(𝜑𝑅 Er 𝑋)    &   (𝜑𝐴𝑅𝐵)       (𝜑 → [𝐴]𝑅 = [𝐵]𝑅)

Theoremecidsn 6153 An equivalence class modulo the identity relation is a singleton. (Contributed by NM, 24-Oct-2004.)
[𝐴] I = {𝐴}

Theoremqseq1 6154 Equality theorem for quotient set. (Contributed by NM, 23-Jul-1995.)
(𝐴 = 𝐵 → (𝐴 / 𝐶) = (𝐵 / 𝐶))

Theoremqseq2 6155 Equality theorem for quotient set. (Contributed by NM, 23-Jul-1995.)
(𝐴 = 𝐵 → (𝐶 / 𝐴) = (𝐶 / 𝐵))

Theoremelqsg 6156* Closed form of elqs 6157. (Contributed by Rodolfo Medina, 12-Oct-2010.)
(𝐵𝑉 → (𝐵 ∈ (𝐴 / 𝑅) ↔ ∃𝑥𝐴 𝐵 = [𝑥]𝑅))

Theoremelqs 6157* Membership in a quotient set. (Contributed by NM, 23-Jul-1995.)
𝐵 ∈ V       (𝐵 ∈ (𝐴 / 𝑅) ↔ ∃𝑥𝐴 𝐵 = [𝑥]𝑅)

Theoremelqsi 6158* Membership in a quotient set. (Contributed by NM, 23-Jul-1995.)
(𝐵 ∈ (𝐴 / 𝑅) → ∃𝑥𝐴 𝐵 = [𝑥]𝑅)

Theoremecelqsg 6159 Membership of an equivalence class in a quotient set. (Contributed by Jeff Madsen, 10-Jun-2010.) (Revised by Mario Carneiro, 9-Jul-2014.)
((𝑅𝑉𝐵𝐴) → [𝐵]𝑅 ∈ (𝐴 / 𝑅))

Theoremecelqsi 6160 Membership of an equivalence class in a quotient set. (Contributed by NM, 25-Jul-1995.) (Revised by Mario Carneiro, 9-Jul-2014.)
𝑅 ∈ V       (𝐵𝐴 → [𝐵]𝑅 ∈ (𝐴 / 𝑅))

Theoremecopqsi 6161 "Closure" law for equivalence class of ordered pairs. (Contributed by NM, 25-Mar-1996.)
𝑅 ∈ V    &   𝑆 = ((𝐴 × 𝐴) / 𝑅)       ((𝐵𝐴𝐶𝐴) → [⟨𝐵, 𝐶⟩]𝑅𝑆)

Theoremqsexg 6162 A quotient set exists. (Contributed by FL, 19-May-2007.) (Revised by Mario Carneiro, 9-Jul-2014.)
(𝐴𝑉 → (𝐴 / 𝑅) ∈ V)

Theoremqsex 6163 A quotient set exists. (Contributed by NM, 14-Aug-1995.)
𝐴 ∈ V       (𝐴 / 𝑅) ∈ V

Theoremuniqs 6164 The union of a quotient set. (Contributed by NM, 9-Dec-2008.)
(𝑅𝑉 (𝐴 / 𝑅) = (𝑅𝐴))

Theoremqsss 6165 A quotient set is a set of subsets of the base set. (Contributed by Mario Carneiro, 9-Jul-2014.) (Revised by Mario Carneiro, 12-Aug-2015.)
(𝜑𝑅 Er 𝐴)       (𝜑 → (𝐴 / 𝑅) ⊆ 𝒫 𝐴)

Theoremuniqs2 6166 The union of a quotient set. (Contributed by Mario Carneiro, 11-Jul-2014.)
(𝜑𝑅 Er 𝐴)    &   (𝜑𝑅𝑉)       (𝜑 (𝐴 / 𝑅) = 𝐴)

Theoremsnec 6167 The singleton of an equivalence class. (Contributed by NM, 29-Jan-1999.) (Revised by Mario Carneiro, 9-Jul-2014.)
𝐴 ∈ V       {[𝐴]𝑅} = ({𝐴} / 𝑅)

Theoremecqs 6168 Equivalence class in terms of quotient set. (Contributed by NM, 29-Jan-1999.)
𝑅 ∈ V       [𝐴]𝑅 = ({𝐴} / 𝑅)

Theoremecid 6169 A set is equal to its converse epsilon coset. (Note: converse epsilon is not an equivalence relation.) (Contributed by NM, 13-Aug-1995.) (Revised by Mario Carneiro, 9-Jul-2014.)
𝐴 ∈ V       [𝐴] E = 𝐴

Theoremecidg 6170 A set is equal to its converse epsilon coset. (Note: converse epsilon is not an equivalence relation.) (Contributed by Jim Kingdon, 8-Jan-2020.)
(𝐴𝑉 → [𝐴] E = 𝐴)

Theoremqsid 6171 A set is equal to its quotient set mod converse epsilon. (Note: converse epsilon is not an equivalence relation.) (Contributed by NM, 13-Aug-1995.) (Revised by Mario Carneiro, 9-Jul-2014.)
(𝐴 / E ) = 𝐴

Theoremectocld 6172* Implicit substitution of class for equivalence class. (Contributed by Mario Carneiro, 9-Jul-2014.)
𝑆 = (𝐵 / 𝑅)    &   ([𝑥]𝑅 = 𝐴 → (𝜑𝜓))    &   ((𝜒𝑥𝐵) → 𝜑)       ((𝜒𝐴𝑆) → 𝜓)

Theoremectocl 6173* Implicit substitution of class for equivalence class. (Contributed by NM, 23-Jul-1995.) (Revised by Mario Carneiro, 9-Jul-2014.)
𝑆 = (𝐵 / 𝑅)    &   ([𝑥]𝑅 = 𝐴 → (𝜑𝜓))    &   (𝑥𝐵𝜑)       (𝐴𝑆𝜓)

Theoremelqsn0m 6174* An element of a quotient set is inhabited. (Contributed by Jim Kingdon, 21-Aug-2019.)
((dom 𝑅 = 𝐴𝐵 ∈ (𝐴 / 𝑅)) → ∃𝑥 𝑥𝐵)

Theoremelqsn0 6175 A quotient set doesn't contain the empty set. (Contributed by NM, 24-Aug-1995.)
((dom 𝑅 = 𝐴𝐵 ∈ (𝐴 / 𝑅)) → 𝐵 ≠ ∅)

Theoremecelqsdm 6176 Membership of an equivalence class in a quotient set. (Contributed by NM, 30-Jul-1995.)
((dom 𝑅 = 𝐴 ∧ [𝐵]𝑅 ∈ (𝐴 / 𝑅)) → 𝐵𝐴)

Theoremxpiderm 6177* A square Cartesian product is an equivalence relation (in general it's not a poset). (Contributed by Jim Kingdon, 22-Aug-2019.)
(∃𝑥 𝑥𝐴 → (𝐴 × 𝐴) Er 𝐴)

Theoremiinerm 6178* The intersection of a nonempty family of equivalence relations is an equivalence relation. (Contributed by Mario Carneiro, 27-Sep-2015.)
((∃𝑦 𝑦𝐴 ∧ ∀𝑥𝐴 𝑅 Er 𝐵) → 𝑥𝐴 𝑅 Er 𝐵)

Theoremriinerm 6179* The relative intersection of a family of equivalence relations is an equivalence relation. (Contributed by Mario Carneiro, 27-Sep-2015.)
((∃𝑦 𝑦𝐴 ∧ ∀𝑥𝐴 𝑅 Er 𝐵) → ((𝐵 × 𝐵) ∩ 𝑥𝐴 𝑅) Er 𝐵)

Theoremerinxp 6180 A restricted equivalence relation is an equivalence relation. (Contributed by Mario Carneiro, 10-Jul-2015.) (Revised by Mario Carneiro, 12-Aug-2015.)
(𝜑𝑅 Er 𝐴)    &   (𝜑𝐵𝐴)       (𝜑 → (𝑅 ∩ (𝐵 × 𝐵)) Er 𝐵)

Theoremecinxp 6181 Restrict the relation in an equivalence class to a base set. (Contributed by Mario Carneiro, 10-Jul-2015.)
(((𝑅𝐴) ⊆ 𝐴𝐵𝐴) → [𝐵]𝑅 = [𝐵](𝑅 ∩ (𝐴 × 𝐴)))

Theoremqsinxp 6182 Restrict the equivalence relation in a quotient set to the base set. (Contributed by Mario Carneiro, 23-Feb-2015.)
((𝑅𝐴) ⊆ 𝐴 → (𝐴 / 𝑅) = (𝐴 / (𝑅 ∩ (𝐴 × 𝐴))))

Theoremqsel 6183 If an element of a quotient set contains a given element, it is equal to the equivalence class of the element. (Contributed by Mario Carneiro, 12-Aug-2015.)
((𝑅 Er 𝑋𝐵 ∈ (𝐴 / 𝑅) ∧ 𝐶𝐵) → 𝐵 = [𝐶]𝑅)

Theoremqliftlem 6184* 𝐹, a function lift, is a subset of 𝑅 × 𝑆. (Contributed by Mario Carneiro, 23-Dec-2016.)
𝐹 = ran (𝑥𝑋 ↦ ⟨[𝑥]𝑅, 𝐴⟩)    &   ((𝜑𝑥𝑋) → 𝐴𝑌)    &   (𝜑𝑅 Er 𝑋)    &   (𝜑𝑋 ∈ V)       ((𝜑𝑥𝑋) → [𝑥]𝑅 ∈ (𝑋 / 𝑅))

Theoremqliftrel 6185* 𝐹, a function lift, is a subset of 𝑅 × 𝑆. (Contributed by Mario Carneiro, 23-Dec-2016.)
𝐹 = ran (𝑥𝑋 ↦ ⟨[𝑥]𝑅, 𝐴⟩)    &   ((𝜑𝑥𝑋) → 𝐴𝑌)    &   (𝜑𝑅 Er 𝑋)    &   (𝜑𝑋 ∈ V)       (𝜑𝐹 ⊆ ((𝑋 / 𝑅) × 𝑌))

Theoremqliftel 6186* Elementhood in the relation 𝐹. (Contributed by Mario Carneiro, 23-Dec-2016.)
𝐹 = ran (𝑥𝑋 ↦ ⟨[𝑥]𝑅, 𝐴⟩)    &   ((𝜑𝑥𝑋) → 𝐴𝑌)    &   (𝜑𝑅 Er 𝑋)    &   (𝜑𝑋 ∈ V)       (𝜑 → ([𝐶]𝑅𝐹𝐷 ↔ ∃𝑥𝑋 (𝐶𝑅𝑥𝐷 = 𝐴)))

Theoremqliftel1 6187* Elementhood in the relation 𝐹. (Contributed by Mario Carneiro, 23-Dec-2016.)
𝐹 = ran (𝑥𝑋 ↦ ⟨[𝑥]𝑅, 𝐴⟩)    &   ((𝜑𝑥𝑋) → 𝐴𝑌)    &   (𝜑𝑅 Er 𝑋)    &   (𝜑𝑋 ∈ V)       ((𝜑𝑥𝑋) → [𝑥]𝑅𝐹𝐴)

Theoremqliftfun 6188* The function 𝐹 is the unique function defined by 𝐹‘[𝑥] = 𝐴, provided that the well-definedness condition holds. (Contributed by Mario Carneiro, 23-Dec-2016.)
𝐹 = ran (𝑥𝑋 ↦ ⟨[𝑥]𝑅, 𝐴⟩)    &   ((𝜑𝑥𝑋) → 𝐴𝑌)    &   (𝜑𝑅 Er 𝑋)    &   (𝜑𝑋 ∈ V)    &   (𝑥 = 𝑦𝐴 = 𝐵)       (𝜑 → (Fun 𝐹 ↔ ∀𝑥𝑦(𝑥𝑅𝑦𝐴 = 𝐵)))

Theoremqliftfund 6189* The function 𝐹 is the unique function defined by 𝐹‘[𝑥] = 𝐴, provided that the well-definedness condition holds. (Contributed by Mario Carneiro, 23-Dec-2016.)
𝐹 = ran (𝑥𝑋 ↦ ⟨[𝑥]𝑅, 𝐴⟩)    &   ((𝜑𝑥𝑋) → 𝐴𝑌)    &   (𝜑𝑅 Er 𝑋)    &   (𝜑𝑋 ∈ V)    &   (𝑥 = 𝑦𝐴 = 𝐵)    &   ((𝜑𝑥𝑅𝑦) → 𝐴 = 𝐵)       (𝜑 → Fun 𝐹)

Theoremqliftfuns 6190* The function 𝐹 is the unique function defined by 𝐹‘[𝑥] = 𝐴, provided that the well-definedness condition holds. (Contributed by Mario Carneiro, 23-Dec-2016.)
𝐹 = ran (𝑥𝑋 ↦ ⟨[𝑥]𝑅, 𝐴⟩)    &   ((𝜑𝑥𝑋) → 𝐴𝑌)    &   (𝜑𝑅 Er 𝑋)    &   (𝜑𝑋 ∈ V)       (𝜑 → (Fun 𝐹 ↔ ∀𝑦𝑧(𝑦𝑅𝑧𝑦 / 𝑥𝐴 = 𝑧 / 𝑥𝐴)))

Theoremqliftf 6191* The domain and range of the function 𝐹. (Contributed by Mario Carneiro, 23-Dec-2016.)
𝐹 = ran (𝑥𝑋 ↦ ⟨[𝑥]𝑅, 𝐴⟩)    &   ((𝜑𝑥𝑋) → 𝐴𝑌)    &   (𝜑𝑅 Er 𝑋)    &   (𝜑𝑋 ∈ V)       (𝜑 → (Fun 𝐹𝐹:(𝑋 / 𝑅)⟶𝑌))

Theoremqliftval 6192* The value of the function 𝐹. (Contributed by Mario Carneiro, 23-Dec-2016.)
𝐹 = ran (𝑥𝑋 ↦ ⟨[𝑥]𝑅, 𝐴⟩)    &   ((𝜑𝑥𝑋) → 𝐴𝑌)    &   (𝜑𝑅 Er 𝑋)    &   (𝜑𝑋 ∈ V)    &   (𝑥 = 𝐶𝐴 = 𝐵)    &   (𝜑 → Fun 𝐹)       ((𝜑𝐶𝑋) → (𝐹‘[𝐶]𝑅) = 𝐵)

Theoremecoptocl 6193* Implicit substitution of class for equivalence class of ordered pair. (Contributed by NM, 23-Jul-1995.)
𝑆 = ((𝐵 × 𝐶) / 𝑅)    &   ([⟨𝑥, 𝑦⟩]𝑅 = 𝐴 → (𝜑𝜓))    &   ((𝑥𝐵𝑦𝐶) → 𝜑)       (𝐴𝑆𝜓)

Theorem2ecoptocl 6194* Implicit substitution of classes for equivalence classes of ordered pairs. (Contributed by NM, 23-Jul-1995.)
𝑆 = ((𝐶 × 𝐷) / 𝑅)    &   ([⟨𝑥, 𝑦⟩]𝑅 = 𝐴 → (𝜑𝜓))    &   ([⟨𝑧, 𝑤⟩]𝑅 = 𝐵 → (𝜓𝜒))    &   (((𝑥𝐶𝑦𝐷) ∧ (𝑧𝐶𝑤𝐷)) → 𝜑)       ((𝐴𝑆𝐵𝑆) → 𝜒)

Theorem3ecoptocl 6195* Implicit substitution of classes for equivalence classes of ordered pairs. (Contributed by NM, 9-Aug-1995.)
𝑆 = ((𝐷 × 𝐷) / 𝑅)    &   ([⟨𝑥, 𝑦⟩]𝑅 = 𝐴 → (𝜑𝜓))    &   ([⟨𝑧, 𝑤⟩]𝑅 = 𝐵 → (𝜓𝜒))    &   ([⟨𝑣, 𝑢⟩]𝑅 = 𝐶 → (𝜒𝜃))    &   (((𝑥𝐷𝑦𝐷) ∧ (𝑧𝐷𝑤𝐷) ∧ (𝑣𝐷𝑢𝐷)) → 𝜑)       ((𝐴𝑆𝐵𝑆𝐶𝑆) → 𝜃)

Theorembrecop 6196* Binary relation on a quotient set. Lemma for real number construction. (Contributed by NM, 29-Jan-1996.)
∈ V    &    Er (𝐺 × 𝐺)    &   𝐻 = ((𝐺 × 𝐺) / )    &    = {⟨𝑥, 𝑦⟩ ∣ ((𝑥𝐻𝑦𝐻) ∧ ∃𝑧𝑤𝑣𝑢((𝑥 = [⟨𝑧, 𝑤⟩] 𝑦 = [⟨𝑣, 𝑢⟩] ) ∧ 𝜑))}    &   ((((𝑧𝐺𝑤𝐺) ∧ (𝐴𝐺𝐵𝐺)) ∧ ((𝑣𝐺𝑢𝐺) ∧ (𝐶𝐺𝐷𝐺))) → (([⟨𝑧, 𝑤⟩] = [⟨𝐴, 𝐵⟩] ∧ [⟨𝑣, 𝑢⟩] = [⟨𝐶, 𝐷⟩] ) → (𝜑𝜓)))       (((𝐴𝐺𝐵𝐺) ∧ (𝐶𝐺𝐷𝐺)) → ([⟨𝐴, 𝐵⟩] [⟨𝐶, 𝐷⟩] 𝜓))

Theoremeroveu 6197* Lemma for eroprf 6199. (Contributed by Jeff Madsen, 10-Jun-2010.) (Revised by Mario Carneiro, 9-Jul-2014.)
𝐽 = (𝐴 / 𝑅)    &   𝐾 = (𝐵 / 𝑆)    &   (𝜑𝑇𝑍)    &   (𝜑𝑅 Er 𝑈)    &   (𝜑𝑆 Er 𝑉)    &   (𝜑𝑇 Er 𝑊)    &   (𝜑𝐴𝑈)    &   (𝜑𝐵𝑉)    &   (𝜑𝐶𝑊)    &   (𝜑+ :(𝐴 × 𝐵)⟶𝐶)    &   ((𝜑 ∧ ((𝑟𝐴𝑠𝐴) ∧ (𝑡𝐵𝑢𝐵))) → ((𝑟𝑅𝑠𝑡𝑆𝑢) → (𝑟 + 𝑡)𝑇(𝑠 + 𝑢)))       ((𝜑 ∧ (𝑋𝐽𝑌𝐾)) → ∃!𝑧𝑝𝐴𝑞𝐵 ((𝑋 = [𝑝]𝑅𝑌 = [𝑞]𝑆) ∧ 𝑧 = [(𝑝 + 𝑞)]𝑇))

Theoremerovlem 6198* Lemma for eroprf 6199. (Contributed by Jeff Madsen, 10-Jun-2010.) (Revised by Mario Carneiro, 30-Dec-2014.)
𝐽 = (𝐴 / 𝑅)    &   𝐾 = (𝐵 / 𝑆)    &   (𝜑𝑇𝑍)    &   (𝜑𝑅 Er 𝑈)    &   (𝜑𝑆 Er 𝑉)    &   (𝜑𝑇 Er 𝑊)    &   (𝜑𝐴𝑈)    &   (𝜑𝐵𝑉)    &   (𝜑𝐶𝑊)    &   (𝜑+ :(𝐴 × 𝐵)⟶𝐶)    &   ((𝜑 ∧ ((𝑟𝐴𝑠𝐴) ∧ (𝑡𝐵𝑢𝐵))) → ((𝑟𝑅𝑠𝑡𝑆𝑢) → (𝑟 + 𝑡)𝑇(𝑠 + 𝑢)))    &    = {⟨⟨𝑥, 𝑦⟩, 𝑧⟩ ∣ ∃𝑝𝐴𝑞𝐵 ((𝑥 = [𝑝]𝑅𝑦 = [𝑞]𝑆) ∧ 𝑧 = [(𝑝 + 𝑞)]𝑇)}       (𝜑 = (𝑥𝐽, 𝑦𝐾 ↦ (℩𝑧𝑝𝐴𝑞𝐵 ((𝑥 = [𝑝]𝑅𝑦 = [𝑞]𝑆) ∧ 𝑧 = [(𝑝 + 𝑞)]𝑇))))

Theoremeroprf 6199* Functionality of an operation defined on equivalence classes. (Contributed by Jeff Madsen, 10-Jun-2010.) (Revised by Mario Carneiro, 30-Dec-2014.)
𝐽 = (𝐴 / 𝑅)    &   𝐾 = (𝐵 / 𝑆)    &   (𝜑𝑇𝑍)    &   (𝜑𝑅 Er 𝑈)    &   (𝜑𝑆 Er 𝑉)    &   (𝜑𝑇 Er 𝑊)    &   (𝜑𝐴𝑈)    &   (𝜑𝐵𝑉)    &   (𝜑𝐶𝑊)    &   (𝜑+ :(𝐴 × 𝐵)⟶𝐶)    &   ((𝜑 ∧ ((𝑟𝐴𝑠𝐴) ∧ (𝑡𝐵𝑢𝐵))) → ((𝑟𝑅𝑠𝑡𝑆𝑢) → (𝑟 + 𝑡)𝑇(𝑠 + 𝑢)))    &    = {⟨⟨𝑥, 𝑦⟩, 𝑧⟩ ∣ ∃𝑝𝐴𝑞𝐵 ((𝑥 = [𝑝]𝑅𝑦 = [𝑞]𝑆) ∧ 𝑧 = [(𝑝 + 𝑞)]𝑇)}    &   (𝜑𝑅𝑋)    &   (𝜑𝑆𝑌)    &   𝐿 = (𝐶 / 𝑇)       (𝜑 :(𝐽 × 𝐾)⟶𝐿)

Theoremeroprf2 6200* Functionality of an operation defined on equivalence classes. (Contributed by Jeff Madsen, 10-Jun-2010.)
𝐽 = (𝐴 / )    &    = {⟨⟨𝑥, 𝑦⟩, 𝑧⟩ ∣ ∃𝑝𝐴𝑞𝐴 ((𝑥 = [𝑝] 𝑦 = [𝑞] ) ∧ 𝑧 = [(𝑝 + 𝑞)] )}    &   (𝜑𝑋)    &   (𝜑 Er 𝑈)    &   (𝜑𝐴𝑈)    &   (𝜑+ :(𝐴 × 𝐴)⟶𝐴)    &   ((𝜑 ∧ ((𝑟𝐴𝑠𝐴) ∧ (𝑡𝐴𝑢𝐴))) → ((𝑟 𝑠𝑡 𝑢) → (𝑟 + 𝑡) (𝑠 + 𝑢)))       (𝜑 :(𝐽 × 𝐽)⟶𝐽)

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