kill(all)$

van=facebook.com/LearningCalculus$

      The term "natural numbers" refers to either the set of: 
      • positive integers 1, 2, 3, ... (OEIS A000027) or
      • nonnegative integers 0, 1, 2, 3, ... (OEIS A001477).
      Regrettably, there seems to be no general agreement about
      whether to include 0 in the set of natural numbers.
      In fact, Ribenboim (1996) states:
      "Let  P be a set of natural numbers;
       whenever convenient, it may be assumed that 0 in P."

f(x,y):=x*(x+y)-y^2-1;

f(1,0);

f(-1,0);