Number of (w,x,y,z) with all terms in {1,...,n} and w<2x and y<=2z.

Terms

a(0) =0a(1) =1a(2) =12a(3) =56a(4) =168a(5) =399a(6) =810a(7) =1480a(8) =2496a(9) =3965a(10) =6000a(11) =8736a(12) =12312a(13) =16891a(14) =22638a(15) =29744a(16) =38400a(17) =48825a(18) =61236a(19) =75880a(20) =93000a(21) =112871a(22) =135762a(23) =161976a(24) =191808a(25) =225589a(26) =263640a(27) =306320a(28) =353976a(29) =406995