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Theorem exists1 2659
Description: Two ways to express "exactly one thing exists". The left-hand side requires only one variable to express this. Both sides are false in set theory, see Theorem dtru 5447. (Contributed by NM, 5-Apr-2004.) (Proof shortened by BJ, 7-Oct-2022.)
Assertion
Ref Expression
exists1 (∃!𝑥 𝑥 = 𝑥 ↔ ∀𝑥 𝑥 = 𝑦)
Distinct variable group:   𝑥,𝑦

Proof of Theorem exists1
StepHypRef Expression
1 equid 2009 . . . 4 𝑥 = 𝑥
21bitru 1546 . . 3 (𝑥 = 𝑥 ↔ ⊤)
32eubii 2583 . 2 (∃!𝑥 𝑥 = 𝑥 ↔ ∃!𝑥⊤)
4 euae 2658 . 2 (∃!𝑥⊤ ↔ ∀𝑥 𝑥 = 𝑦)
53, 4bitri 275 1 (∃!𝑥 𝑥 = 𝑥 ↔ ∀𝑥 𝑥 = 𝑦)
Colors of variables: wff setvar class
Syntax hints:  wb 206  wal 1535  wtru 1538  ∃!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
This theorem depends on definitions:  df-bi 207  df-an 396  df-tru 1540  df-ex 1777  df-mo 2538  df-eu 2567
This theorem is referenced by:  exists2  2660
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