Half the number of 3 X 3 0..n symmetric arrays with no 2 X 2 subblock summing to 2n.

Terms

a(0) =13a(1) =192a(2) =1320a(3) =5470a(4) =17499a(5) =45892a(6) =105856a(7) =219564a(8) =421825a(9) =758560a(10) =1296408a(11) =2117882a(12) =3336655a(13) =5087460a(14) =7549184a(15) =10927192a(16) =15486741a(17) =21525024a(18) =29417800a(19) =39578070a(20) =52519203a(21) =68796772a(22) =89091840a(23) =114132100a(24) =144799369a(25) =182025792