$($Section $2\mathrm{.}3)13.$ Determine whether each of these functions from $Z$ to $Z$ is onto.

a$)f(n)=n-1$

b$)f(n)=n^2+1$

c$)f(n)=n^3$

d$)f(n)=|$~$\frac{n}{2}$~$|($This means the ceiling of $\frac{n}{2},$ so the closest integer rounded up$.)$