Diagonal sums of number triangle A046521.

Terms

a(0) =1a(1) =2a(2) =7a(3) =26a(4) =101a(5) =402a(6) =1625a(7) =6638a(8) =27319a(9) =113054a(10) =469811a(11) =1958706a(12) =8187063a(13) =34290934a(14) =143864999a(15) =604402050a(16) =2542083509a(17) =10702020746a(18) =45090876913a(19) =190110250998a(20) =801997354525