while (n > 9) n = sum_digits(n); return (n == 3 || n == 6 || n == 9);

clort on Nov 21, 2020 | parent | context | favorite | on: Test for Divisibility by 13

  while (n > 9)
    n = sum_digits(n);

  return (n == 3 || n == 6 || n == 9);