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) =15a(3) =69a(4) =203a(5) =467a(6) =935a(7) =1661a(8) =2770a(9) =4326a(10) =6476a(11) =9280a(12) =13025a(13) =17635a(14) =23475a(15) =30614a(16) =39319a(17) =49557a(18) =61969a(19) =76227a(20) =93166a(21) =112525a(22) =134801a(23) =159969a(24) =189233a(25) =221589a(26) =258185a(27) =299022a(28) =344875a(29) =395057