Metamath Proof Explorer


Theorem salpreimagtlt

Description: If all the preimages of lef-open, unbounded above intervals, belong to a sigma-algebra, then all the preimages of right-open, unbounded below intervals, belong to the sigma-algebra. (iii) implies (i) in Proposition 121B of Fremlin1 p. 36. (Contributed by Glauco Siliprandi, 26-Jun-2021)

Ref Expression
Hypotheses salpreimagtlt.x x φ
salpreimagtlt.a a φ
salpreimagtlt.s φ S SAlg
salpreimagtlt.u A = S
salpreimagtlt.b φ x A B *
salpreimagtlt.p φ a x A | a < B S
salpreimagtlt.c φ C
Assertion salpreimagtlt φ x A | B < C S

Proof

Step Hyp Ref Expression
1 salpreimagtlt.x x φ
2 salpreimagtlt.a a φ
3 salpreimagtlt.s φ S SAlg
4 salpreimagtlt.u A = S
5 salpreimagtlt.b φ x A B *
6 salpreimagtlt.p φ a x A | a < B S
7 salpreimagtlt.c φ C
8 nfv x a
9 1 8 nfan x φ a
10 nfv b φ a
11 3 adantr φ a S SAlg
12 5 adantlr φ a x A B *
13 nfv a b
14 2 13 nfan a φ b
15 nfv a x A | b < B S
16 14 15 nfim a φ b x A | b < B S
17 eleq1w a = b a b
18 17 anbi2d a = b φ a φ b
19 breq1 a = b a < B b < B
20 19 rabbidv a = b x A | a < B = x A | b < B
21 20 eleq1d a = b x A | a < B S x A | b < B S
22 18 21 imbi12d a = b φ a x A | a < B S φ b x A | b < B S
23 16 22 6 chvarfv φ b x A | b < B S
24 23 adantlr φ a b x A | b < B S
25 simpr φ a a
26 9 10 11 12 24 25 salpreimagtge φ a x A | a B S
27 1 2 3 4 5 26 7 salpreimagelt φ x A | B < C S