a(n) = Sum_{d|n} (n/d)^(d-1) * binomial(d+n-1,n).

Terms

a(0) =1a(1) =4a(2) =11a(3) =46a(4) =127a(5) =596a(6) =1717a(7) =7792a(8) =24806a(9) =108450a(10) =352717a(11) =1563914a(12) =5200301a(13) =22539046a(14) =77876117a(15) =331982444a(16) =1166803111a(17) =4945693769a(18) =17672631901a(19) =74053888812a(20) =269344740908