Arrays of dumbbells.

Terms

a(0) =1a(1) =4a(2) =11a(3) =26a(4) =56a(5) =114a(6) =223a(7) =424a(8) =789a(9) =1444a(10) =2608a(11) =4660a(12) =8253a(13) =14508a(14) =25343a(15) =44030a(16) =76136a(17) =131110a(18) =224955a(19) =384720a(20) =656041a(21) =1115784a(22) =1893216a(23) =3205416a(24) =5416441a(25) =9136084a(26) =15384563a(27) =25866914a(28) =43429784a(29) =72821274