Signature

The signature of a knot, σ(K), is equal to σ(V + V^t), the signature of V + V^t where V is a Seifert matrix for the knot and V^t is its transpose. The knot's signature also equals the signature of the intersection form on H₂(W), where W is the two-fold branched cover of D⁴ over a pushed-in Seifert surface.

The signature provides bounds for the smooth 4-genus of a knot; for instance, slice knots have signature zero.