a(n) = Product_{k=0..n} (4*k+1)^(n-k).A263430a(n) = Product_{k=0..n} (4*k+1)^(n-k).Terms a(0) =1a(1) =1a(2) =5a(3) =225a(4) =131625a(5) =1309010625External referencesoeis: A263430