### From Homeworkwiki

**Let f be a subset of Z X Z defined by f {ab, (a + b) : a, b ∈ Z}. Is f a function from Z into Z.**

**Justify your answers.**

**Solution:** We observe that:

1 X 6 = 6 and 2 X 3 = 6

=>(1 X 6, 1 + 6) ∈ f and (2 X 3, 2 + 3) ∈ f

=>(6, 7) ∈ f and (6,5) ∈ f

So, f is not a function from Z to Z.