reserve r,s,t,u for Real;

theorem Th61:
  for X being LinearTopSpace, A,B being Subset of X holds Int A +
  Int B c= Int(A+B)
proof
  let X be LinearTopSpace, A,B be Subset of X;
  Int A c= A & Int B c= B by TOPS_1:16;
  then Int A + Int B c= A+B by Th11;
  hence thesis by TOPS_1:24;
end;
