let n be Nat; for R being Subset of (TOP-REAL n)
for p being Point of (TOP-REAL n)
for P being Subset of (TOP-REAL n) st R is connected & R is open & P = { q where q is Point of (TOP-REAL n) : ( q <> p & q in R & ( for f being Function of I[01],(TOP-REAL n) holds
( not f is continuous or not rng f c= R or not f . 0 = p or not f . 1 = q ) ) ) } holds
P is open
let R be Subset of (TOP-REAL n); for p being Point of (TOP-REAL n)
for P being Subset of (TOP-REAL n) st R is connected & R is open & P = { q where q is Point of (TOP-REAL n) : ( q <> p & q in R & ( for f being Function of I[01],(TOP-REAL n) holds
( not f is continuous or not rng f c= R or not f . 0 = p or not f . 1 = q ) ) ) } holds
P is open
let p be Point of (TOP-REAL n); for P being Subset of (TOP-REAL n) st R is connected & R is open & P = { q where q is Point of (TOP-REAL n) : ( q <> p & q in R & ( for f being Function of I[01],(TOP-REAL n) holds
( not f is continuous or not rng f c= R or not f . 0 = p or not f . 1 = q ) ) ) } holds
P is open
let P be Subset of (TOP-REAL n); ( R is connected & R is open & P = { q where q is Point of (TOP-REAL n) : ( q <> p & q in R & ( for f being Function of I[01],(TOP-REAL n) holds
( not f is continuous or not rng f c= R or not f . 0 = p or not f . 1 = q ) ) ) } implies P is open )
assume that
A1:
( R is connected & R is open )
and
A2:
P = { q where q is Point of (TOP-REAL n) : ( q <> p & q in R & ( for f being Function of I[01],(TOP-REAL n) holds
( not f is continuous or not rng f c= R or not f . 0 = p or not f . 1 = q ) ) ) }
; P is open
A3:
TopStruct(# the carrier of (TOP-REAL n), the topology of (TOP-REAL n) #) = TopSpaceMetr (Euclid n)
by EUCLID:def 8;
then reconsider P9 = P as Subset of (TopSpaceMetr (Euclid n)) ;
A4:
P c= R
now for u being Point of (Euclid n) st u in P holds
ex r being Real st
( r > 0 & Ball (u,r) c= P9 )A5:
TopStruct(# the
carrier of
(TOP-REAL n), the
topology of
(TOP-REAL n) #)
= TopSpaceMetr (Euclid n)
by EUCLID:def 8;
then reconsider R9 =
R as
Subset of
(TopSpaceMetr (Euclid n)) ;
let u be
Point of
(Euclid n);
( u in P implies ex r being Real st
( r > 0 & Ball (u,r) c= P9 ) )reconsider p2 =
u as
Point of
(TOP-REAL n) by TOPREAL3:8;
assume A6:
u in P
;
ex r being Real st
( r > 0 & Ball (u,r) c= P9 )
R9 is
open
by A1, A5, PRE_TOPC:30;
then consider r being
Real such that A7:
r > 0
and A8:
Ball (
u,
r)
c= R9
by A4, A6, TOPMETR:15;
take r =
r;
( r > 0 & Ball (u,r) c= P9 )thus
r > 0
by A7;
Ball (u,r) c= P9reconsider r9 =
r as
Real ;
A9:
p2 in Ball (
u,
r9)
by A7, TBSP_1:11;
Ball (
u,
r)
c= P9
hence
Ball (
u,
r)
c= P9
;
verum end;
then
P9 is open
by TOPMETR:15;
hence
P is open
by A3, PRE_TOPC:30; verum