Knot Genus

The least genus of any Seifert surface for a given knot. The unknot is the only knot with genus 0.

Usually, one denotes by g(K) the genus of the knot K. The knot genus has the pleasing additivity property that if K_1 and K_2 are oriented knots, then


where the sum on the left hand side denotes knot sum. This additivity implies immediately, by induction, that any oriented knot can be factored into a sum of prime knots. Indeed, by the additivity of knot genus, any knot of genus 1 is prime. Furthermore, given any knot K of genus g(K)>1, either K itself is prime, or K can be written as a sum of knots of lesser genus, each of which can be decomposed into a sum of prime knots, by induction.

A nonobvious fact is that the prime decomposition is also unique.

Portions of this entry contributed by Rasmus Hedegaard

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Lickorish, W. B. R. Ch. 2 in An Introduction to Knot Theory. New York: Springer-Verlag, 1997.

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Knot Genus

Cite this as:

Hedegaard, Rasmus and Weisstein, Eric W. "Knot Genus." From MathWorld--A Wolfram Web Resource.

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