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A036648 Number of centered 5-valent trees with n nodes.

%I A036648 #19 Dec 26 2020 11:08:57
%S A036648 0,1,0,1,1,2,3,6,11,24,48,109,242,574,1346,3258,7928,19664,49158,
%T A036648 124384,316791,813231,2099326,5451613,14226697,37306971,98247737,
%U A036648 259779698,689385447,1835644498,4902992215,13133825317,35276818036
%N A036648 Number of centered 5-valent trees with n nodes.
%H A036648 E. M. Rains and N. J. A. Sloane, <a href="https://cs.uwaterloo.ca/journals/JIS/cayley.html">On Cayley's Enumeration of Alkanes (or 4-Valent Trees)</a>, J. Integer Sequences, Vol. 2 (1999), Article 99.1.1.
%H A036648 <a href="/index/Tra#trees">Index entries for sequences related to trees</a>
%F A036648 a(n) = A036650(n) - A036649(n).
%t A036648 n = 30; (* algorithm from Rains and Sloane *)
%t A036648 S4[f_,h_,x_] := f[h,x]^4/24 + f[h,x]^2 f[h,x^2]/4 + f[h,x] f[h,x^3]/3 + f[h,x^2]^2/8 + f[h,x^4]/4;
%t A036648 S5[f_,h_,x_] := f[h,x]^5/120 + f[h,x]^3 f[h,x^2]/12 + f[h,x]^2 f[h,x^3]/6 + f[h,x] f[h,x^2]^2/8 + f[h,x] f[h,x^4]/4 + f[h,x^2] f[h,x^3]/6 + f[h,x^5]/5;
%t A036648 T[-1,z_] := 1;  T[h_,z_] := T[h,z] = Table[z^k, {k,0,n}].Take[CoefficientList[z^(n+1) + 1 + S4[T,h-1,z]z, z], n+1];
%t A036648 Sum[Take[CoefficientList[z^(n+1) + S5[T,h-1,z]z - S5[T,h-2,z]z - (T[h-1,z] - T[h-2,z]) (T[h-1,z]-1),z], n+1], {h,1,n/2}] + PadRight[{0,1}, n+1] (* _Robert A. Russell_, Sep 15 2018 *)
%Y A036648 Cf. A036649, A036650.
%K A036648 nonn
%O A036648 0,6
%A A036648 _N. J. A. Sloane_