theorem Th18:
  a>=1 & n>=1 implies n-root a >= 1 & a >= n-root a
proof
  assume that
A1: a>=1 and
A2: n>=1;
   n -Root a >= 1 & a >= n -Root a by A1,A2,PREPOWER:29;
  hence thesis by A2,Def1;
end;
