Sum of product of odd numbers <= n and the product of even numbers <= n.

Terms

a(0) =2a(1) =3a(2) =5a(3) =11a(4) =23a(5) =63a(6) =153a(7) =489a(8) =1329a(9) =4785a(10) =14235a(11) =56475a(12) =181215a(13) =780255a(14) =2672145a(15) =12348945a(16) =44781345a(17) =220253985a(18) =840523635a(19) =4370620275a(20) =17465201775a(21) =95498916975a(22) =397983749625