a(n) = A027082(n, 2n).

Terms

a(0) =1a(1) =1a(2) =2a(3) =5a(4) =11a(5) =27a(6) =68a(7) =175a(8) =458a(9) =1215a(10) =3258a(11) =8815a(12) =24031a(13) =65937a(14) =181936a(15) =504473a(16) =1404879a(17) =3927495a(18) =11017802a(19) =31004871a(20) =87497297a(21) =247559933a(22) =702089528a(23) =1995483853a(24) =5682959332a(25) =16214622573a(26) =46343364290a(27) =132667706857a(28) =380358924209