Alternating sum of double factorials: n!! - (n-1)!! + (n-2)!! - ... 1!!.

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =9a(5) =39a(6) =66a(7) =318a(8) =627a(9) =3213a(10) =7182a(11) =38898a(12) =96237a(13) =548883a(14) =1478142a(15) =8843778a(16) =25615647a(17) =160178913a(18) =494550162a(19) =3221341038a(20) =10527969537a(21) =71221636863a(22) =245012506362