Number of nX3 binary arrays with each 1 adjacent to exactly two other 1s.

Terms

a(0) =1a(1) =3a(2) =6a(3) =10a(4) =19a(5) =37a(6) =69a(7) =129a(8) =244a(9) =460a(10) =865a(11) =1629a(12) =3069a(13) =5779a(14) =10882a(15) =20494a(16) =38595a(17) =72681a(18) =136873a(19) =257761a(20) =485416a(21) =914136a(22) =1721505a(23) =3241945a(24) =6105241a(25) =11497411a(26) =21651966a(27) =40775058a(28) =76787731a(29) =144606925