Definition df-2ndc 22591

Description: Define the class of all second-countable topologies. (Contributed by Jeff Hankins, 17-Jan-2010.)

Assertion

Ref	Expression
df-2ndc	⊢ 2ndω = {𝑗 ∣ ∃𝑥 ∈ TopBases (𝑥 ≼ ω ∧ (topGen‘𝑥) = 𝑗)}

Distinct variable group:   𝑥,𝑗

Detailed syntax breakdown of Definition df-2ndc

Step	Hyp	Ref	Expression
1		c2ndc 22589	. 2 class 2ndω
2		vx	. . . . . . 7 setvar 𝑥
3	2	cv 1538	. . . . . 6 class 𝑥
4		com 7712	. . . . . 6 class ω
5		cdom 8731	. . . . . 6 class ≼
6	3, 4, 5	wbr 5074	. . . . 5 wff 𝑥 ≼ ω
7		ctg 17148	. . . . . . 7 class topGen
8	3, 7	cfv 6433	. . . . . 6 class (topGen‘𝑥)
9		vj	. . . . . . 7 setvar 𝑗
10	9	cv 1538	. . . . . 6 class 𝑗
11	8, 10	wceq 1539	. . . . 5 wff (topGen‘𝑥) = 𝑗
12	6, 11	wa 396	. . . 4 wff (𝑥 ≼ ω ∧ (topGen‘𝑥) = 𝑗)
13		ctb 22095	. . . 4 class TopBases
14	12, 2, 13	wrex 3065	. . 3 wff ∃𝑥 ∈ TopBases (𝑥 ≼ ω ∧ (topGen‘𝑥) = 𝑗)
15	14, 9	cab 2715	. 2 class {𝑗 ∣ ∃𝑥 ∈ TopBases (𝑥 ≼ ω ∧ (topGen‘𝑥) = 𝑗)}
16	1, 15	wceq 1539	1 wff 2ndω = {𝑗 ∣ ∃𝑥 ∈ TopBases (𝑥 ≼ ω ∧ (topGen‘𝑥) = 𝑗)}

This definition is referenced by:  is2ndc  22597