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Theorem dfnul3 4249
 Description: Alternate definition of the empty set. (Contributed by NM, 25-Mar-2004.)
Assertion
Ref Expression
dfnul3 ∅ = {𝑥𝐴 ∣ ¬ 𝑥𝐴}

Proof of Theorem dfnul3
StepHypRef Expression
1 pm3.24 406 . . . . 5 ¬ (𝑥𝐴 ∧ ¬ 𝑥𝐴)
2 equid 2019 . . . . 5 𝑥 = 𝑥
31, 22th 267 . . . 4 (¬ (𝑥𝐴 ∧ ¬ 𝑥𝐴) ↔ 𝑥 = 𝑥)
43con1bii 360 . . 3 𝑥 = 𝑥 ↔ (𝑥𝐴 ∧ ¬ 𝑥𝐴))
54abbii 2866 . 2 {𝑥 ∣ ¬ 𝑥 = 𝑥} = {𝑥 ∣ (𝑥𝐴 ∧ ¬ 𝑥𝐴)}
6 dfnul2 4248 . 2 ∅ = {𝑥 ∣ ¬ 𝑥 = 𝑥}
7 df-rab 3118 . 2 {𝑥𝐴 ∣ ¬ 𝑥𝐴} = {𝑥 ∣ (𝑥𝐴 ∧ ¬ 𝑥𝐴)}
85, 6, 73eqtr4i 2834 1 ∅ = {𝑥𝐴 ∣ ¬ 𝑥𝐴}
 Colors of variables: wff setvar class Syntax hints:  ¬ wn 3   ∧ wa 399   = wceq 1538   ∈ wcel 2112  {cab 2779  {crab 3113  ∅c0 4246 This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1797  ax-4 1811  ax-5 1911  ax-6 1970  ax-7 2015  ax-9 2122  ax-ext 2773 This theorem depends on definitions:  df-bi 210  df-an 400  df-ex 1782  df-sb 2070  df-clab 2780  df-cleq 2794  df-rab 3118  df-dif 3887  df-nul 4247 This theorem is referenced by:  difidALT  4288  kmlem3  9567
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