# Signature Function and Nullity

The Tristram-Levine signature function of a knot, σ(K), is equal to
σ((1- ω)V + (1- ω)^{*}V^{t}), where V is a Seifert
Matrix for the knot and ω is the unit complex number, exp(π i t). The nullity at ω is the nullity of the matrix.

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

The numeric values of the jump points shown in the graphs are rough
numerical estimates. More precise values can be computed as they are the roots of the Alexander
polynomial.

Each signature function is given as a vector as in the example:

11n_75 {{0.2300534562, {0, 1, 2}, 1}, {0.3333333333, {2, 2, 2}, 2}}

This indicates that the Alexander polynomial has two roots on the unit circle, numbers A and B with t values roughly .23 and .33. At A the signture jumps from 0 to 2, and at A it equals 1. The nullity at A is 1. Similarly B the signture jumps is constant near, and at B. The nullity at B is 1.

The following example is one for which the averaged signature function is trivial but the signature function is not trivial. This is an example of a slice knot with nontrivial signature function. The signature function obstructs this knot from being doubly slice.

12n_56 {{0.3333333333, {0, 1, 0}, 1}}

## References

[1] Levine, J., "Invariants of knot cobordism," Invent. Math. 8 (1969), 98-110.

[2] Tristram, A. "Some cobordism invariants for links," Proc. Camb. Phil. Soc. 66 (1969), 251-264.