a(n) = (2n+1)!! + (2n)!! where k!! = A006882(k).

A306185

a(n) = (2n+1)!! + (2n)!! where k!! = A006882(k).

Terms

    a(0) =5a(1) =23a(2) =153a(3) =1329a(4) =14235a(5) =181215a(6) =2672145a(7) =44781345a(8) =840523635a(9) =17465201775a(10) =397983749625

External references