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

Terms

a(0) =0a(1) =1a(2) =13a(3) =62a(4) =187a(5) =444a(6) =885a(7) =1616a(8) =2677a(9) =4227a(10) =6326a(11) =9163a(12) =12742a(13) =17468a(14) =23167a(15) =30278a(16) =38876a(17) =49268a(18) =61315a(19) =75858a(20) =92363a(21) =111867a(22) =134039a(23) =159426a(24) =187841a(25) =220810a(26) =257093a(27) =297936a(28) =343312a(29) =394178