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Theorem 2eu8 2657
Description: Two equivalent expressions for double existential uniqueness. Curiously, we can put ∃! on either of the internal conjuncts but not both. We can also commute ∃!𝑥∃!𝑦 using 2eu7 2656. Usage of this theorem is discouraged because it depends on ax-13 2375. (Contributed by NM, 20-Feb-2005.) (New usage is discouraged.)
Assertion
Ref Expression
2eu8 (∃!𝑥∃!𝑦(∃𝑥𝜑 ∧ ∃𝑦𝜑) ↔ ∃!𝑥∃!𝑦(∃!𝑥𝜑 ∧ ∃𝑦𝜑))

Proof of Theorem 2eu8
StepHypRef Expression
1 2eu2 2651 . . 3 (∃!𝑥𝑦𝜑 → (∃!𝑦∃!𝑥𝜑 ↔ ∃!𝑦𝑥𝜑))
21pm5.32i 574 . 2 ((∃!𝑥𝑦𝜑 ∧ ∃!𝑦∃!𝑥𝜑) ↔ (∃!𝑥𝑦𝜑 ∧ ∃!𝑦𝑥𝜑))
3 nfeu1 2586 . . . . 5 𝑥∃!𝑥𝜑
43nfeu 2592 . . . 4 𝑥∃!𝑦∃!𝑥𝜑
54euan 2619 . . 3 (∃!𝑥(∃!𝑦∃!𝑥𝜑 ∧ ∃𝑦𝜑) ↔ (∃!𝑦∃!𝑥𝜑 ∧ ∃!𝑥𝑦𝜑))
6 ancom 460 . . . . . 6 ((∃!𝑥𝜑 ∧ ∃𝑦𝜑) ↔ (∃𝑦𝜑 ∧ ∃!𝑥𝜑))
76eubii 2583 . . . . 5 (∃!𝑦(∃!𝑥𝜑 ∧ ∃𝑦𝜑) ↔ ∃!𝑦(∃𝑦𝜑 ∧ ∃!𝑥𝜑))
8 nfe1 2148 . . . . . 6 𝑦𝑦𝜑
98euan 2619 . . . . 5 (∃!𝑦(∃𝑦𝜑 ∧ ∃!𝑥𝜑) ↔ (∃𝑦𝜑 ∧ ∃!𝑦∃!𝑥𝜑))
10 ancom 460 . . . . 5 ((∃𝑦𝜑 ∧ ∃!𝑦∃!𝑥𝜑) ↔ (∃!𝑦∃!𝑥𝜑 ∧ ∃𝑦𝜑))
117, 9, 103bitri 297 . . . 4 (∃!𝑦(∃!𝑥𝜑 ∧ ∃𝑦𝜑) ↔ (∃!𝑦∃!𝑥𝜑 ∧ ∃𝑦𝜑))
1211eubii 2583 . . 3 (∃!𝑥∃!𝑦(∃!𝑥𝜑 ∧ ∃𝑦𝜑) ↔ ∃!𝑥(∃!𝑦∃!𝑥𝜑 ∧ ∃𝑦𝜑))
13 ancom 460 . . 3 ((∃!𝑥𝑦𝜑 ∧ ∃!𝑦∃!𝑥𝜑) ↔ (∃!𝑦∃!𝑥𝜑 ∧ ∃!𝑥𝑦𝜑))
145, 12, 133bitr4ri 304 . 2 ((∃!𝑥𝑦𝜑 ∧ ∃!𝑦∃!𝑥𝜑) ↔ ∃!𝑥∃!𝑦(∃!𝑥𝜑 ∧ ∃𝑦𝜑))
15 2eu7 2656 . 2 ((∃!𝑥𝑦𝜑 ∧ ∃!𝑦𝑥𝜑) ↔ ∃!𝑥∃!𝑦(∃𝑥𝜑 ∧ ∃𝑦𝜑))
162, 14, 153bitr3ri 302 1 (∃!𝑥∃!𝑦(∃𝑥𝜑 ∧ ∃𝑦𝜑) ↔ ∃!𝑥∃!𝑦(∃!𝑥𝜑 ∧ ∃𝑦𝜑))
Colors of variables: wff setvar class
Syntax hints:  wb 206  wa 395  wex 1776  ∃!weu 2566
This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1792  ax-4 1806  ax-5 1908  ax-6 1965  ax-7 2005  ax-10 2139  ax-11 2155  ax-12 2175  ax-13 2375
This theorem depends on definitions:  df-bi 207  df-an 396  df-or 848  df-tru 1540  df-ex 1777  df-nf 1781  df-mo 2538  df-eu 2567
This theorem is referenced by: (None)
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