G.f.: Product_{k>=1} 1/(1-x^k)^(4*k).

Terms

a(0) =1a(1) =4a(2) =18a(3) =64a(4) =215a(5) =660a(6) =1938a(7) =5400a(8) =14527a(9) =37728a(10) =95278a(11) =234344a(12) =563506a(13) =1326796a(14) =3066040a(15) =6963048a(16) =15564661a(17) =34282360a(18) =74486376a(19) =159785472a(20) =338703796a(21) =709957616a(22) =1472529670a(23) =3023894672a(24) =6151408852a(25) =12402137024a(26) =24792822174a(27) =49162962280a(28) =96737562642