Ample line bundles on blown up surfaces

Abstract

Given a smooth complex projective surface $S$ and an ample divisor $H$ on $S$, consider the blow up of $S$ along $k$ points in general position. Let $H'$ be the pullback of $H$ and $E_1,..., E_k$ be the exceptional divisors. We show that $L = nH' -E_1 - ... -E_k$ is ample if and only if $L^2$ is positive provided the integer $n$ is at least 3.