a(1) = 1; a(n+1) = -a(n) + 2 * Sum_{d|n} a(n/d) * a(d).

Terms

a(0) =1a(1) =1a(2) =3a(3) =9a(4) =29a(5) =87a(6) =273a(7) =819a(8) =2493a(9) =7497a(10) =22607a(11) =67821a(12) =203919a(13) =611757a(14) =1836363a(15) =5509437a(16) =16531749a(17) =49595247a(18) =148796757a(19) =446390271a(20) =1339201845a(21) =4017608811a(22) =12052916861a(23) =36158750583a(24) =108476535993a(25) =325429609661a(26) =976289644659