a(n) = (1/9)*((6*n - 7)*2^(n-1) - (-1)^n).

Terms

a(0) =0a(1) =1a(2) =5a(3) =15a(4) =41a(5) =103a(6) =249a(7) =583a(8) =1337a(9) =3015a(10) =6713a(11) =14791a(12) =32313a(13) =70087a(14) =151097a(15) =324039a(16) =691769a(17) =1470919a(18) =3116601a(19) =6582727a(20) =13864505a(21) =29127111a(22) =61050425a(23) =127693255a(24) =266571321a(25) =555512263a(26) =1155763769a(27) =2401006023a(28) =4980969017a(29) =10319851975