a(0) = a(1) = 1; a(n) = a(n-1) + (n+1)*a(n-2).

Terms

a(0) =1a(1) =1a(2) =4a(3) =8a(4) =28a(5) =76a(6) =272a(7) =880a(8) =3328a(9) =12128a(10) =48736a(11) =194272a(12) =827840a(13) =3547648a(14) =15965248a(15) =72727616a(16) =344136832a(17) =1653233920a(18) =8191833728a(19) =41256512128a(20) =213285020416