Number of compositions of n when parts 1 and 2 are of two kinds.

Terms

a(0) =1a(1) =2a(2) =6a(3) =17a(4) =49a(5) =141a(6) =406a(7) =1169a(8) =3366a(9) =9692a(10) =27907a(11) =80355a(12) =231373a(13) =666212a(14) =1918281a(15) =5523470a(16) =15904198a(17) =45794313a(18) =131859469a(19) =379674209a(20) =1093228314a(21) =3147825473a(22) =9063802210a(23) =26098178316a(24) =75146709475a(25) =216376326215a(26) =623030800329