Metamath Proof Explorer


Theorem salpreimagelt

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

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

Proof

Step Hyp Ref Expression
1 salpreimagelt.x x φ
2 salpreimagelt.a a φ
3 salpreimagelt.s φ S SAlg
4 salpreimagelt.u A = S
5 salpreimagelt.b φ x A B *
6 salpreimagelt.p φ a x A | a B S
7 salpreimagelt.c φ C
8 4 eqcomi S = A
9 8 a1i φ S = A
10 9 difeq1d φ S x A | C B = A x A | C B
11 7 rexrd φ C *
12 1 5 11 preimagelt φ A x A | C B = x A | B < C
13 10 12 eqtr2d φ x A | B < C = S x A | C B
14 7 ancli φ φ C
15 nfcv _ a C
16 15 nfel1 a C
17 2 16 nfan a φ C
18 nfv a x A | C B S
19 17 18 nfim a φ C x A | C B S
20 eleq1 a = C a C
21 20 anbi2d a = C φ a φ C
22 breq1 a = C a B C B
23 22 rabbidv a = C x A | a B = x A | C B
24 23 eleq1d a = C x A | a B S x A | C B S
25 21 24 imbi12d a = C φ a x A | a B S φ C x A | C B S
26 19 25 6 vtoclg1f C φ C x A | C B S
27 7 14 26 sylc φ x A | C B S
28 3 27 saldifcld φ S x A | C B S
29 13 28 eqeltrd φ x A | B < C S