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

Terms

a(0) =0a(1) =1a(2) =11a(3) =50a(4) =150a(5) =355a(6) =721a(7) =1316a(8) =2220a(9) =3525a(10) =5335a(11) =7766a(12) =10946a(13) =15015a(14) =20125a(15) =26440a(16) =34136a(17) =43401a(18) =54435a(19) =67450a(20) =82670a(21) =100331a(22) =120681a(23) =143980a(24) =170500a(25) =200525a(26) =234351a(27) =272286a(28) =314650a(29) =361775