Sum of n!!, with n>=0.

Terms

a(0) =1a(1) =2a(2) =4a(3) =7a(4) =15a(5) =30a(6) =78a(7) =183a(8) =567a(9) =1512a(10) =5352a(11) =15747a(12) =61827a(13) =196962a(14) =842082a(15) =2869107a(16) =13191027a(17) =47650452a(18) =233445012a(19) =888174087a(20) =4604065287a(21) =18353375862a(22) =100102982262a(23) =416337125487