a(n) = Sum_{k=1..n} (2k-1)!.A138523a(n) = Sum_{k=1..n} (2k-1)!.Terms a(0) =1a(1) =7a(2) =127a(3) =5167a(4) =368047a(5) =40284847a(6) =6267305647External referencesoeis: A138523