Number of n X 2 zero-sum -1..1 arrays with rows and columns lexicographically nondecreasing.

Terms

a(0) =2a(1) =7a(2) =21a(3) =56a(4) =130a(5) =281a(6) =555a(7) =1034a(8) =1827a(9) =3090a(10) =5028a(11) =7929a(12) =12143a(13) =18144a(14) =26512a(15) =37981a(16) =53440a(17) =74001a(18) =100965a(19) =135932a(20) =180770a(21) =237705a(22) =309317a(23) =398656a(24) =509191a(25) =644982a(26) =810632a(27) =1011433a(28) =1253353a(29) =1543214