reserve x,x0, r, s, h for Real,

  n for Element of NAT,
  rr, y for set,
  Z for open Subset of REAL,

  f, f1, f2 for PartFunc of REAL,REAL;

theorem Th11:
  rng arctan = ].-PI/2,PI/2.[
proof
  dom (tan|].-PI/2,PI/2.[) = ].-PI/2,PI/2.[ by Th1,RELAT_1:62;
  hence thesis by FUNCT_1:33;
end;
