On the graded quotients of the ring of Fricke characters of a free group
  First, we study a descending filtration of the ring of Fricke characters of a free group consisting of ideals on which the automorphism group of a free group acts. Then using it, we define a descending filtration of the IA-automorphism group of a free group, and investigate a relation between it and the Andreadakis-Johnson filtration. 
Eri Hatakenaka and Takao Satoh
math.GR