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Theorem 2eu4 2585
 Description: This theorem provides us with a definition of double existential uniqueness ("exactly one 𝑥 and exactly one 𝑦"). Naively one might think (incorrectly) that it could be defined by ∃!𝑥∃!𝑦𝜑. See 2eu1 2582 for a condition under which the naive definition holds and 2exeu 2578 for a one-way implication. See 2eu5 2586 and 2eu8 2589 for alternate definitions. (Contributed by NM, 3-Dec-2001.) (Proof shortened by Wolf Lammen, 14-Sep-2019.)
Assertion
Ref Expression
2eu4 ((∃!𝑥𝑦𝜑 ∧ ∃!𝑦𝑥𝜑) ↔ (∃𝑥𝑦𝜑 ∧ ∃𝑧𝑤𝑥𝑦(𝜑 → (𝑥 = 𝑧𝑦 = 𝑤))))
Distinct variable groups:   𝑥,𝑦,𝑧,𝑤   𝜑,𝑧,𝑤
Allowed substitution hints:   𝜑(𝑥,𝑦)

Proof of Theorem 2eu4
StepHypRef Expression
1 eu5 2524 . . 3 (∃!𝑥𝑦𝜑 ↔ (∃𝑥𝑦𝜑 ∧ ∃*𝑥𝑦𝜑))
2 eu5 2524 . . . 4 (∃!𝑦𝑥𝜑 ↔ (∃𝑦𝑥𝜑 ∧ ∃*𝑦𝑥𝜑))
3 excom 2082 . . . . 5 (∃𝑦𝑥𝜑 ↔ ∃𝑥𝑦𝜑)
43anbi1i 731 . . . 4 ((∃𝑦𝑥𝜑 ∧ ∃*𝑦𝑥𝜑) ↔ (∃𝑥𝑦𝜑 ∧ ∃*𝑦𝑥𝜑))
52, 4bitri 264 . . 3 (∃!𝑦𝑥𝜑 ↔ (∃𝑥𝑦𝜑 ∧ ∃*𝑦𝑥𝜑))
61, 5anbi12i 733 . 2 ((∃!𝑥𝑦𝜑 ∧ ∃!𝑦𝑥𝜑) ↔ ((∃𝑥𝑦𝜑 ∧ ∃*𝑥𝑦𝜑) ∧ (∃𝑥𝑦𝜑 ∧ ∃*𝑦𝑥𝜑)))
7 anandi 888 . 2 ((∃𝑥𝑦𝜑 ∧ (∃*𝑥𝑦𝜑 ∧ ∃*𝑦𝑥𝜑)) ↔ ((∃𝑥𝑦𝜑 ∧ ∃*𝑥𝑦𝜑) ∧ (∃𝑥𝑦𝜑 ∧ ∃*𝑦𝑥𝜑)))
8 2mo2 2579 . . 3 ((∃*𝑥𝑦𝜑 ∧ ∃*𝑦𝑥𝜑) ↔ ∃𝑧𝑤𝑥𝑦(𝜑 → (𝑥 = 𝑧𝑦 = 𝑤)))
98anbi2i 730 . 2 ((∃𝑥𝑦𝜑 ∧ (∃*𝑥𝑦𝜑 ∧ ∃*𝑦𝑥𝜑)) ↔ (∃𝑥𝑦𝜑 ∧ ∃𝑧𝑤𝑥𝑦(𝜑 → (𝑥 = 𝑧𝑦 = 𝑤))))
106, 7, 93bitr2i 288 1 ((∃!𝑥𝑦𝜑 ∧ ∃!𝑦𝑥𝜑) ↔ (∃𝑥𝑦𝜑 ∧ ∃𝑧𝑤𝑥𝑦(𝜑 → (𝑥 = 𝑧𝑦 = 𝑤))))
 Colors of variables: wff setvar class Syntax hints:   → wi 4   ↔ wb 196   ∧ wa 383  ∀wal 1521  ∃wex 1744  ∃!weu 2498  ∃*wmo 2499 This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1762  ax-4 1777  ax-5 1879  ax-6 1945  ax-7 1981  ax-10 2059  ax-11 2074  ax-12 2087 This theorem depends on definitions:  df-bi 197  df-or 384  df-an 385  df-ex 1745  df-nf 1750  df-eu 2502  df-mo 2503 This theorem is referenced by:  2eu5  2586  2eu6  2587
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