Tohoku Mathematical Journal 2017 March SECOND SERIES VOL. 69, NO. 1

 Tohoku Math. J. 69 (2017), 67-84

Title ON THE UNIVERSAL DEFORMATIONS FOR ${\rm SL}_2$-REPRESENTATIONS OF KNOT GROUPS

Author Masanori Morishita, Yu Takakura, Yuji Terashima and Jun Ueki

Abstract. Based on the analogies between knot theory and number theory, we study a deformation theory for ${\rm SL}_2$-representations of knot groups, following after Mazur's deformation theory of Galois representations. Firstly, by employing the pseudo-${\rm SL}_2$-representations, we prove the existence of the universal deformation of a given ${\rm SL}_2$-representation of a finitely generated group $\Pi$ over a perfect field $k$ whose characteristic is not 2. We then show its connection with the character scheme for ${\rm SL}_2$-representations of $\Pi$ when $k$ is an algebraically closed field. We investigate examples concerning Riley representations of 2-bridge knot groups and give explicit forms of the universal deformations. Finally we discuss the universal deformation of the holonomy representation of a hyperbolic knot group in connection with Thurston's theory on deformations of hyperbolic structures.