theorem
  f is continuous & H is open implies for F st F=("f).:H holds F is open
proof
  assume that
A1: f is continuous and
A2: H is open;
  let F such that
A3: F=("f).:H;
  let X be Subset of T;
  assume X in F;
  then consider y being object such that
A4: y in dom "f and
A5: y in H and
A6: X=("f).y by A3,FUNCT_1:def 6;
  reconsider Y=y as Subset of S by A5;
A7: X = f"Y by A4,A6,FUNCT_3:21;
A8: [#]S={} implies [#]T={};
  Y is open by A2,A5;
  hence thesis by A1,A8,A7,Th43;
end;
