a(n) = A005248(n) - n.

Terms

a(0) =2a(1) =2a(2) =5a(3) =15a(4) =43a(5) =118a(6) =316a(7) =836a(8) =2199a(9) =5769a(10) =15117a(11) =39592a(12) =103670a(13) =271430a(14) =710633a(15) =1860483a(16) =4870831a(17) =12752026a(18) =33385264a(19) =87403784a(20) =228826107a(21) =599074557a(22) =1568397585a(23) =4106118220a(24) =10749957098a(25) =28143753098a(26) =73681302221a(27) =192900153591