Abstract

The concordance crosscap number ${\gamma }_c(K)$ of a knot $K$ is the smallest crosscap number ${\gamma }_3(K^{\prime })$ of any knot $K^{\prime }$ concordant to $K$ (and with ${\gamma }_3(K^{\prime })$ defined as the least first Betti number of any nonorientable surface $\mathrm{\Sigma }$ embedded in $S^3$ with boundary $K^{\prime }$). This invariant has been introduced and studied by Zhang (2007) using knot determinants and signatures, and has further been studied by Livingston (2008) using the Alexander polynomial. We show in this work that the rational Witt class of a knot can be used to obtain a lower bound on the concordance crosscap number, by means of a new integer-valued invariant we call the Witt span of the knot.

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