It seems easy but I can't prove it. Can anyone give proof or reference?

If $A$ is a Seifert matrix for $K$ and $\omega \in \mathbb{C}$ has norm 1, then the TristramLevine signature $\sigma_\omega(K)$ is the signature of the matrix $(1\omega)A + (1\bar{\omega})A^T = (1\bar{\omega})(A^T  \omega A),$ which jumps when some eigenvalue of $A^T  \omega A$ crosses zero (i.e. changes sign). At these values of $\omega$ the product of the eigenvalues, which is $\det(A^T\omega A) = \Delta_K(\omega)$, must therefore be zero. 

