  The sequence 2,5,15,51,187,... with the form $(2^n+1)(2^{n-1}+1)/3$ has two
interpretations in terms of the density of a language with four letters and the
cardinality of the quotient of $\ZZ_2^n\times \ZZ_2^n$ under the action of the
special linear group $\op{SL}(2,\ZZ)$. The last interpretation follows the rank
of the fundamental group of the $\ZZ_2^n$-cobordism category in dimension 1+1.
This article presents how to pass from one side to another between these two
approaches.
