From George Gilbert's marks problem: jumping 3 marks at a time (initial positions).

Terms

a(0) =2a(1) =22a(2) =38a(3) =114a(4) =219a(5) =354a(6) =3035a(7) =6182a(8) =6502a(9) =11935a(10) =22946a(11) =33122a(12) =58975a(13) =137394a(14) =865634a(15) =1060511a(16) =2604630a(17) =3041138a(18) =13633887a(19) =30516651a(20) =51664754a(21) =73000543a(22) =126052390a(23) =469204879a(24) =12224092843a(25) =14479084326a(26) =19560684367