reserve n,k,b for Nat, i for Integer;

theorem
  41 divides n iff 41 divides value(digits(n,10)/^1,10) - 4*digits(n,10).0
  proof
    digits(n,10)/^1=mid(digits(n,10),2,len(digits(n,10))) by Th3;
    hence thesis by Th58;
  end;
