reserve a,b,i,k,m,n for Nat;
reserve s,z for non zero Nat;
reserve c for Complex;

theorem
  13^2+1 = 7^2 + 11^2 & 17^2+1 = 11^2 + 13^2 &
  23^2+1 = 13^2 + 19^2 & 31^2+1 = 11^2 + 29^2;
