Almost Positive Links Have Negative Signature

We analyze properties of links which have diagrams with a small number of negative crossings. We show that if a nontrivial link has a diagram with all crossings positive except possibly one, then the signature of the link is negative. If a link diagram has two negative crossings, we show that the signature of the link is nonpositive with the exception of the left-handed Hopf link (with possible trivial components). We also characterize those links which have signature zero and diagrams with two negative crossings. In particular, we show that if a nontrivial knot has a diagram with two negative crossings then the signature of the knot is negative, unless the knot is a twist knot with negative clasp. We completely determine all trivial link diagrams with two or fewer negative crossings. For a knot diagram with three negative crossings, the signature of the knot is nonpositive except the left-handed trefoil knot. These results generalize those of L. Rudolph, T. Cochran, E. Gompf, P. Traczyk, and J. H. Przytycki, solve Conjecture 5 of [P-2], and give a partial answer to Problem 2.8 of [Co-G] about knots dominating the trefoil knot or the trivial knot. We also describe all unknotting number one positive knots.