a(n) = ((4*n)!! - (4*n-1)!!)/(4*n+1).A273983a(n) = ((4*n)!! - (4*n-1)!!)/(4*n+1).Terms a(0) =1a(1) =31a(2) =2745a(3) =487935a(4) =145769625a(5) =65830256415External referencesoeis: A273983