Quote:

No polynomial $\displaystyle f (n)$ with integral coefficients, not a constant, can be prime for all $\displaystyle n$, or for all sufficiently large $\displaystyle n$.

We may assume that the leading coefficient in $\displaystyle f(n)$ is positive, so that $\displaystyle f(n)\rightarrow\infty$ when $\displaystyle n\rightarrow\infty$, and $\displaystyle f(n) > 1$ for $\displaystyle n > N$, say. If $\displaystyle x > N$ and $\displaystyle f(x)=a_0x^k+\ldots =y>1$ then $\displaystyle f(ry+x)=a_0\left(ry+x\right)^k+\ldots$ is divisible by $\displaystyle y$ for every integral $\displaystyle r$; and $\displaystyle f(ry+x)$ tends to infinity with $\displaystyle r$. Hence there are infinitely many composite values of $\displaystyle f(n)$.