a(n) = A028491(n) - 1.

Terms

a(0) =2a(1) =6a(2) =12a(3) =70a(4) =102a(5) =540a(6) =1090a(7) =1366a(8) =1626a(9) =4176a(10) =9010a(11) =9550a(12) =36912a(13) =43062a(14) =49680a(15) =57916a(16) =483610a(17) =877842a(18) =2215302a(19) =2704980a(20) =3598866a(21) =7973130a(22) =8530116