Quadrinomial coefficients.

Terms

a(0) =1a(1) =1a(2) =3a(3) =10a(4) =31a(5) =101a(6) =336a(7) =1128a(8) =3823a(9) =13051a(10) =44803a(11) =154518a(12) =534964a(13) =1858156a(14) =6472168a(15) =22597760a(16) =79067375a(17) =277164295a(18) =973184313a(19) =3422117190a(20) =12049586631a(21) =42478745781a(22) =149915252028a(23) =529606271560