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# Crosscap number

In the mathematical field of knot theory, the crosscap number of a knot K is the minimum of

${\displaystyle 1-\chi (S),\,}$

taken over all compact, connected, non-orientable surfaces S bounding K; here ${\displaystyle \chi }$ is the Euler characteristic. The crosscap number of the unknot is zero, as the Euler characteristic of the disk is one.

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• Concordance Crosscap Number of a Knot (part 1/3)
• Crosscap Sliceform
• Energy of a knot part 1/4

## Examples

The formula for the knot sum is

${\displaystyle C(k_{1})+C(k_{2})-1\leq C(k_{1}\#k_{2})\leq C(k_{1})+C(k_{2}).\,}$

• Clark, B.E. "Crosscaps and Knots", Int. J. Math and Math. Sci, Vol 1, 1978, pp 113–124
• Murakami, Hitoshi and Yasuhara, Akira. "Crosscap number of a knot," Pacific J. Math. 171 (1995), no. 1, 261–273.
• Teragaito, Masakazu. "Crosscap numbers of torus knots," Topology Appl. 138 (2004), no. 1–3, 219–238.
• Teragaito, Masakazu and Hirasawa, Mikami. "Crosscap numbers of 2-bridge knots," Arxiv:math.GT/0504446.
• J.Uhing. "Zur Kreuzhaubenzahl von Knoten", diploma thesis, 1997, University of Dortmund, (German language)
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