Arrays of dumbbells.

Terms

a(0) =1a(1) =10a(2) =56a(3) =234a(4) =815a(5) =2504a(6) =7018a(7) =18336a(8) =45328a(9) =107160a(10) =244198a(11) =539656a(12) =1161987a(13) =2446906a(14) =5054440a(15) =10266850a(16) =20549117a(17) =40595568a(18) =79271188a(19) =153190480a(20) =293278496a(21) =556737696a(22) =1048772300a(23) =1961855408a(24) =3646420325a(25) =6737649754