Arrays of dumbbells.

Terms

a(0) =1a(1) =13a(2) =92a(3) =473a(4) =1982a(5) =7191a(6) =23431a(7) =70234a(8) =196941a(9) =522939a(10) =1327002a(11) =3240917a(12) =7660538a(13) =17602967a(14) =39466363a(15) =86593478a(16) =186399956a(17) =394478234a(18) =822229746a(19) =1690521204a(20) =3433033150a(21) =6893852746a(22) =13702694284a(23) =26982983126a(24) =52680389239