a(1) = a(2) = 1; a(n) = sigma(a(n-1)+a(n-2)).

Terms

a(0) =1a(1) =1a(2) =3a(3) =7a(4) =18a(5) =31a(6) =57a(7) =180a(8) =320a(9) =1092a(10) =2478a(11) =10368a(12) =25704a(13) =100800a(14) =393120a(15) =1965600a(16) =10326624a(17) =39657600a(18) =131580288a(19) =511223790a(20) =1469266560a(21) =4911616440a(22) =22120488000a(23) =95841111600a(24) =460020244320