Roland Bacher, Apr 08 2008, Table of n, a(n) for n = 0..14
R. Bacher, The special subgroup of invertible non-commutative rational power series as a metric group, arXiv:0804.1092
The reference contains a somewhat complicated formula.
Sequence in context: A078296 A239164 A209176 * A133461 A246981 A174745
Adjacent sequences: A137340 A137341 A137342 * A137344 A137345 A137346
Roland Bacher, Apr 08 2008