Number of Baxter matrices of size 3 X n.

Terms

a(0) =1a(1) =14a(2) =69a(3) =203a(4) =463a(5) =903a(6) =1585a(7) =2579a(8) =3963a(9) =5823a(10) =8253a(11) =11355a(12) =15239a(13) =20023a(14) =25833a(15) =32803a(16) =41075a(17) =50799a(18) =62133a(19) =75243a(20) =90303a(21) =107495a(22) =127009a(23) =149043a(24) =173803a(25) =201503a(26) =232365a(27) =266619a(28) =304503a(29) =346263