Partial sums of A000340, second partial sums of A003462.

Terms

a(0) =1a(1) =6a(2) =24a(3) =82a(4) =261a(5) =804a(6) =2440a(7) =7356a(8) =22113a(9) =66394a(10) =199248a(11) =597822a(12) =1793557a(13) =5380776a(14) =16142448a(15) =48427480a(16) =145282593a(17) =435847950a(18) =1307544040a(19) =3922632330a(20) =11767897221a(21) =35303691916a(22) =105911076024a(23) =317733228372a(24) =953199685441