Dual support vectors and margin degeneracy
= Dual support vectors and margin degeneracy
A dual <support vector> has $\alpha_i>0$. Every such point has signed margin at most one, but a point exactly on the margin can have $\alpha_i=0$ in a degenerate solution. If $\alpha_i=0$, complementary slackness forces $\xi_i=0$, so the point is correctly classified with margin at least one.