Sum_{k=1..n} k!! * (n+1-k)!!, where k!! = k*(k-2)*(k-4)..*(1 or 2).

Terms

a(0) =0a(1) =1a(2) =4a(3) =10a(4) =28a(5) =71a(6) =204a(7) =556a(8) =1716a(9) =5049a(10) =16884a(11) =53418a(12) =193500a(13) =654639a(14) =2560140a(15) =9209016a(16) =38697300a(17) =147193425a(18) =661035060a(19) =2645445870a(20) =12627252540a(21) =52926443415a(22) =267088884300