Arrays of dumbbells.

Terms

a(0) =1a(1) =7a(2) =29a(3) =94a(4) =263a(5) =667a(6) =1577a(7) =3538a(8) =7622a(9) =15900a(10) =32314a(11) =64274a(12) =125561a(13) =241569a(14) =458715a(15) =861242a(16) =1601081a(17) =2950693a(18) =5396209a(19) =9801012a(20) =17692092a(21) =31759800a(22) =56727588a(23) =100861716a(24) =178585489a(25) =314995915a(26) =553650761a(27) =969967510a(28) =1694235803