a(n) = Sum_{j=0..2*n} Sum_{k=0..j} A026536(j, k).

Terms

a(0) =1a(1) =6a(2) =35a(3) =204a(4) =1199a(5) =7089a(6) =42070a(7) =250269a(8) =1491262a(9) =8896310a(10) =53118352a(11) =317373194a(12) =1897253203a(13) =11346582851a(14) =67882263130a(15) =406231442387