let Y be non trivial T_0-TopSpace; :: thesis: ( not Y is T_1 implies Sierpinski_Space is_Retract_of Y )
given p, q being Point of Y such that A1: p <> q and
A2: for W, V being Subset of Y st W is open & V is open & p in W & not q in W & q in V holds
p in V ; :: according to URYSOHN1:def 7 :: thesis:
( ex V being Subset of Y st
( V is open & p in V & not q in V ) or ex W being Subset of Y st
( W is open & not p in W & q in W ) ) by ;
then consider V being Subset of Y such that
A3: V is open and
A4: ( ( p in V & not q in V ) or ( not p in V & q in V ) ) ;
A5: TopStruct(# the carrier of (), the topology of () #) = TopStruct(# the carrier of Y, the topology of Y #) by WAYBEL25:def 2;
then consider x, y being Element of () such that
A6: ( ( p in V & not q in V & x = q & y = p ) or ( not p in V & q in V & x = p & y = q ) ) by A4;
now :: thesis: for W being open Subset of () st x in W holds
y in W
let W be open Subset of (); :: thesis: ( x in W implies y in W )
W is open Subset of Y by ;
hence ( x in W implies y in W ) by A2, A3, A6; :: thesis: verum
end;
then (0,1) --> (x,y) is continuous Function of Sierpinski_Space,() by YELLOW16:47;
then reconsider i = (0,1) --> (x,y) as continuous Function of Sierpinski_Space,Y by ;
reconsider V = V as open Subset of () by ;
reconsider c = chi (V, the carrier of Y) as continuous Function of Y,Sierpinski_Space by ;
c * i = id Sierpinski_Space by ;
hence Sierpinski_Space is_Retract_of Y by WAYBEL25:def 1; :: thesis: verum