Delta-Unknotting Number for Pretzel Knots

Abstract

The $\Delta$-unknotting number for a knot is defined as the minimum number of $\Delta$-moves needed to deform the knot into the trivial knot. It is known that, for positive pretzel knots, the $\Delta$-unknotting number coincides with the second coefficient of their Conway polynomial. In this paper, we compute the $\Delta$-unknotting number for positive pretzel knots. As a consequence of the above result, among positive pretzel knots of odd type with a fixed crossing number $n$, where $n$ is odd, the $\Delta$-unknotting number is maximized by $P(1,1,...,1)(\cong T(2,n))$, and the maximum value is $\frac{1}{8}(n^2-1)$. We also obtain a similar result for torus knots. We further determine the $\Delta$-unknotting number for pretzel knots of type $P(-1,p_2,...,p_n)$, where $p_i$ is a positive odd integer for $2\le i\le n$ and $n$ is odd.

18 pages, 7 figures. Substantial revision: added Theorem 1.6 through Conjecture 1.14, and added Sections 5 and 6