a(0) = a(1) = 1; a(n) = a(n-2) + Sum_{k=0..n-2} binomial(n-2,k) * a(k).

Terms

a(0) =1a(1) =1a(2) =2a(3) =3a(4) =7a(5) =16a(6) =43a(7) =123a(8) =384a(9) =1283a(10) =4575a(11) =17294a(12) =69013a(13) =289613a(14) =1273934a(15) =5856811a(16) =28070535a(17) =139936316a(18) =724141487a(19) =3882776711a(20) =21536499372a(21) =123388080843a(22) =729195916303