Number of independent vertex sets in the n-helm graph.

Terms

a(0) =3a(1) =4a(2) =12a(3) =28a(4) =72a(5) =184a(6) =480a(7) =1264a(8) =3360a(9) =8992a(10) =24192a(11) =65344a(12) =177024a(13) =480640a(14) =1307136a(15) =3559168a(16) =9699840a(17) =26452480a(18) =72173568a(19) =196989952a(20) =537802752a(21) =1468536832a(22) =4010582016a(23) =10954043392a(24) =29920862208a(25) =81733033984a(26) =223274237952a(27) =609947435008