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

Terms

a(0) =0a(1) =0a(2) =3a(3) =17a(4) =55a(5) =135a(6) =280a(7) =518a(8) =882a(9) =1410a(10) =2145a(11) =3135a(12) =4433a(13) =6097a(14) =8190a(15) =10780a(16) =13940a(17) =17748a(18) =22287a(19) =27645a(20) =33915a(21) =41195a(22) =49588a(23) =59202a(24) =70150a(25) =82550a(26) =96525a(27) =112203a(28) =129717a(29) =149205