Abstract: H. Furusho proved the beautiful result that of the three defining equations for associators, the pentagon implies the two hexagons. In this paper we present a simpler proof for this theorem (although our paper is less dense and hence only slightly shorter). In particular, we package the use of algebraic geometry and Groethendieck-Teichmüller groups into a useful and previously known principle, and, less significantly, we eliminate the use of spherical braids.
[Dr1]V.
G. Drinfel′d, Quasi-Hopf algebras, Algebra i Analiz
1 (1989), no. 6, 114–148 (Russian); English
transl., Leningrad Math. J. 1 (1990), no. 6,
1419–1457. MR 1047964
(91b:17016)
[Dr2]V.
G. Drinfel′d, On quasitriangular quasi-Hopf algebras and on a
group that is closely connected with
𝐺𝑎𝑙(\𝑜𝑣𝑒𝑟𝑙𝑖𝑛𝑒{𝑄}/𝑄),
Algebra i Analiz 2 (1990), no. 4, 149–181
(Russian); English transl., Leningrad Math. J. 2 (1991),
no. 4, 829–860. MR 1080203
(92f:16047)