a(n) = a(n-3) + A001654(n-1) with a(0)=0, a(1)=0 and a(2)=1.

Terms

a(0) =0a(1) =0a(2) =1a(3) =2a(4) =6a(5) =16a(6) =42a(7) =110a(8) =289a(9) =756a(10) =1980a(11) =5184a(12) =13572a(13) =35532a(14) =93025a(15) =243542a(16) =637602a(17) =1669264a(18) =4370190a(19) =11441306a(20) =29953729a(21) =78419880a(22) =205305912a(23) =537497856a(24) =1407187656a(25) =3684065112a(26) =9645007681a(27) =25250957930a(28) =66107866110