A228513 - OEIS

%I #10 Jun 10 2018 11:25:27

%S 1,3,16,152,2448,61232,2204416,108016512,6913057024,559957619456,

%T 55995761946624,6775487195543552,975670156158275584,

%U 164888256390748581888,32318098252586722066432,7271572106832012464979968,1861522459348995191034937344,537979990751859610209097023488

%N a(n) = Sum_{k=0..n} 2^k*(n!/k!)^2.

%C Generally, Sum_{k=0..n} x^k*(n!/k!)^2 is asymptotic to BesselI(0,2*sqrt(x))*(n!)^2

%F a(n) = (n^2+2)*a(n-1) - 2*(n-1)^2*a(n-2).

%F a(n) ~ 2*Pi*BesselI(0,2*sqrt(2)) * n^(2*n+1)/exp(2*n).

%t Table[(n!)^2*Sum[2^j/(j!)^2, {j, 0, n}], {n, 0, 20}]

%t Total/@Table[2^k (n!/k!)^2,{n,0,20},{k,0,n}] (* _Harvey P. Dale_, Jun 10 2018 *)

%Y Cf. A000522, A006040, A217284.

%K nonn,easy

%O 0,2

%A _Vaclav Kotesovec_, Aug 24 2013