Quadrinomial coefficients.

Terms

a(0) =1a(1) =2a(2) =6a(3) =20a(4) =65a(5) =216a(6) =728a(7) =2472a(8) =8451a(9) =29050a(10) =100298a(11) =347568a(12) =1208220a(13) =4211312a(14) =14712960a(15) =51507280a(16) =180642391a(17) =634551606a(18) =2232223626a(19) =7862669700a(20) =27727507521a(21) =97884558992a(22) =345891702456