Two-Special Pairs in a Free Group.

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =1a(6) =2a(7) =6a(8) =14a(9) =30a(10) =62a(11) =129a(12) =252a(13) =499a(14) =984a(15) =1884a(16) =3624a(17) =7025a(18) =13358a(19) =25694a(20) =49316