theorem Th30:
  I c= sqrt I
  proof
    let x be object;
    assume
A1: x in I; then
    reconsider x as Element of A;
    x|^1 in I by A1,BINOM:8; then
    x in {a where a is Element of A: ex n being
      Element of NAT st a|^n in I};
    hence thesis by IDEAL_1:def 24;
  end;
