Number of 3 X 3 matrices of nonnegative integer entries with all row and column sums <= n.

Terms

a(0) =1a(1) =34a(2) =451a(3) =3380a(4) =17531a(5) =70466a(6) =235014a(7) =679722a(8) =1757085a(9) =4147792a(10) =9084361a(11) =18683314a(12) =36421463a(13) =67798940a(14) =121239308a(15) =209285436a(16) =350158809a(17) =569759574a(18) =904194895a(19) =1402934104a(20) =2132700691a(21) =3182223374a(22) =4667981330a(23) =6741092150a(24) =9595505205a(25) =13477677876a(26) =18697927509a(27) =25643668006a(28) =34794756655