let TM be metrizable TopSpace; for Null being Subset of TM st TM | Null is second-countable holds
( ( Null is with_finite_small_inductive_dimension & ind Null <= 0 ) iff for p being Point of TM
for U being open Subset of TM st p in U holds
ex W being open Subset of TM st
( p in W & W c= U & Null misses Fr W ) )
let Null be Subset of TM; ( TM | Null is second-countable implies ( ( Null is with_finite_small_inductive_dimension & ind Null <= 0 ) iff for p being Point of TM
for U being open Subset of TM st p in U holds
ex W being open Subset of TM st
( p in W & W c= U & Null misses Fr W ) ) )
assume A1:
TM | Null is second-countable
; ( ( Null is with_finite_small_inductive_dimension & ind Null <= 0 ) iff for p being Point of TM
for U being open Subset of TM st p in U holds
ex W being open Subset of TM st
( p in W & W c= U & Null misses Fr W ) )
hereby ( ( for p being Point of TM
for U being open Subset of TM st p in U holds
ex W being open Subset of TM st
( p in W & W c= U & Null misses Fr W ) ) implies ( Null is with_finite_small_inductive_dimension & ind Null <= 0 ) )
assume A2:
(
Null is
with_finite_small_inductive_dimension &
ind Null <= 0 )
;
for p being Point of TM
for U being open Subset of TM st p in U holds
ex W being open Subset of TM st
( p in W & W c= U & Null misses Fr W )let p be
Point of
TM;
for U being open Subset of TM st p in U holds
ex W being open Subset of TM st
( p in W & W c= U & Null misses Fr W )let U be
open Subset of
TM;
( p in U implies ex W being open Subset of TM st
( p in W & W c= U & Null misses Fr W ) )assume A3:
p in U
;
ex W being open Subset of TM st
( p in W & W c= U & Null misses Fr W )reconsider P =
{p} as
Subset of
TM by A3, ZFMISC_1:31;
not
p in U `
by A3, XBOOLE_0:def 5;
then A4:
P misses U `
by ZFMISC_1:50;
not
TM is
empty
by A3;
then consider L being
Subset of
TM such that A5:
L separates P,
U `
and A6:
L misses Null
by A1, A2, A4, Th37;
consider W,
W1 being
open Subset of
TM such that A7:
P c= W
and A8:
U ` c= W1
and A9:
W misses W1
and A10:
L = (W \/ W1) `
by A5, METRIZTS:def 3;
take W =
W;
( p in W & W c= U & Null misses Fr W )
(
W c= W1 ` &
W1 ` c= (U `) ` )
by A8, A9, SUBSET_1:12, SUBSET_1:23;
hence
(
p in W &
W c= U )
by A7, XBOOLE_1:1, ZFMISC_1:31;
Null misses Fr Wthus
Null misses Fr W
verum
end;
set TN = TM | Null;
assume A15:
for p being Point of TM
for U being open Subset of TM st p in U holds
ex W being open Subset of TM st
( p in W & W c= U & Null misses Fr W )
; ( Null is with_finite_small_inductive_dimension & ind Null <= 0 )
A16:
for p being Point of (TM | Null)
for U being open Subset of (TM | Null) st p in U holds
ex W being open Subset of (TM | Null) st
( p in W & W c= U & Fr W is with_finite_small_inductive_dimension & ind (Fr W) <= 0 - 1 )
proof
let p be
Point of
(TM | Null);
for U being open Subset of (TM | Null) st p in U holds
ex W being open Subset of (TM | Null) st
( p in W & W c= U & Fr W is with_finite_small_inductive_dimension & ind (Fr W) <= 0 - 1 )let U be
open Subset of
(TM | Null);
( p in U implies ex W being open Subset of (TM | Null) st
( p in W & W c= U & Fr W is with_finite_small_inductive_dimension & ind (Fr W) <= 0 - 1 ) )
assume A17:
p in U
;
ex W being open Subset of (TM | Null) st
( p in W & W c= U & Fr W is with_finite_small_inductive_dimension & ind (Fr W) <= 0 - 1 )
A18:
[#] (TM | Null) = Null
by PRE_TOPC:def 5;
then
p in Null
by A17;
then reconsider p9 =
p as
Point of
TM ;
consider U9 being
Subset of
TM such that A19:
U9 is
open
and A20:
U = U9 /\ the
carrier of
(TM | Null)
by TSP_1:def 1;
p9 in U9
by A17, A20, XBOOLE_0:def 4;
then consider W9 being
open Subset of
TM such that A21:
(
p9 in W9 &
W9 c= U9 )
and A22:
Null misses Fr W9
by A15, A19;
reconsider W =
W9 /\ the
carrier of
(TM | Null) as
Subset of
(TM | Null) by XBOOLE_1:17;
reconsider W =
W as
open Subset of
(TM | Null) by TSP_1:def 1;
take
W
;
( p in W & W c= U & Fr W is with_finite_small_inductive_dimension & ind (Fr W) <= 0 - 1 )
thus
(
p in W &
W c= U )
by A17, A20, A21, XBOOLE_0:def 4, XBOOLE_1:26;
( Fr W is with_finite_small_inductive_dimension & ind (Fr W) <= 0 - 1 )
A23:
(Fr W9) /\ Null = {}
by A22, XBOOLE_0:def 7;
Fr W c= (Fr W9) /\ Null
by A18, Th1;
hence
(
Fr W is
with_finite_small_inductive_dimension &
ind (Fr W) <= 0 - 1 )
by A23, Th6;
verum
end;
then A24:
TM | Null is with_finite_small_inductive_dimension
by Th15;
then A25:
Null is with_finite_small_inductive_dimension
by Th18;
ind (TM | Null) <= 0
by A16, A24, Th16;
hence
( Null is with_finite_small_inductive_dimension & ind Null <= 0 )
by A25, Lm5; verum