a(n) = Sum_{i=0..2*n} Sum_{j=0..n-1} A026519(j, i).

Terms

a(0) =1a(1) =4a(2) =10a(3) =28a(4) =64a(5) =172a(6) =388a(7) =1036a(8) =2332a(9) =6220a(10) =13996a(11) =37324a(12) =83980a(13) =223948a(14) =503884a(15) =1343692a(16) =3023308a(17) =8062156a(18) =18139852a(19) =48372940a(20) =108839116a(21) =290237644a(22) =653034700a(23) =1741425868a(24) =3918208204a(25) =10448555212a(26) =23509249228a(27) =62691331276