let X be RealNormSpace; :: thesis: for V being Subset of
for Vt being Subset of st V = Vt holds
( V is open iff Vt is open )
let V be Subset of ; :: thesis: for Vt being Subset of st V = Vt holds
( V is open iff Vt is open )
let Vt be Subset of ; :: thesis: ( V = Vt implies ( V is open iff Vt is open ) )
reconsider VVt = Vt as Subset of by Def4;
assume A1: V = Vt ; :: thesis: ( V is open iff Vt is open )
( Vt is open iff VVt is open ) by Th20;
hence ( V is open iff Vt is open ) by A1, Th16; :: thesis: verum