Number of (w,x,y,z) with all terms in {1,...,n} and w <= x > y <= z.

Terms

a(0) =0a(1) =0a(2) =4a(3) =21a(4) =65a(5) =155a(6) =315a(7) =574a(8) =966a(9) =1530a(10) =2310a(11) =3355a(12) =4719a(13) =6461a(14) =8645a(15) =11340a(16) =14620a(17) =18564a(18) =23256a(19) =28785a(20) =35245a(21) =42735a(22) =51359a(23) =61226a(24) =72450a(25) =85150a(26) =99450a(27) =115479a(28) =133371a(29) =153265