The function f: defined by f(x) = 7x is onto because for any real number r, we have that r/7 is a

The function f: ℝ → ℝ defined by f(x) = 7x is onto because for any real number r, we have that r/7 is a real number and f(r/7) = r. Consider the same function defined on the integers g: ℤ → ℤ by g(n) = 7n. Explain why g is not onto ℤ and give one integer that g cannot output.