Mathematical Research Letters
Volume 11 (2004)
Even sets of four nodes on rational surfaces
Pages: 799 – 808
We describe smooth rational projective algebraic surfaces $X$, over an algebraically closed field of characteristic different from $2$, having an even set of four disjoint $(-2)$-curves $N_1,\ldots,N_4$, i.e. such that $N_1+\cdots+N_4$ is divisible by 2 in $\Pic(X)$.