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%I #16 Sep 06 2023 01:57:09
%S 1,420,44310,2107140,57671880,1041949020,13577591370,135798494460,
%T 1091481669390,7302195414780,41800229045610,209294745286620,
%U 933146517188760,3759426449644740,13853429338548630,47171740235162340
%N Coordination sequence for A_20 lattice.
%H R. Bacher, P. de la Harpe and B. Venkov, <a href="http://dx.doi.org/10.1016/S0764-4442(97)83542-2">Séries de croissance et séries d'Ehrhart associées aux réseaux de racines</a>, C. R. Acad. Sci. Paris, 325 (Series 1) (1997), 1137-1142.
%H J. H. Conway and N. J. A. Sloane, Low-Dimensional Lattices VII: Coordination Sequences, Proc. Royal Soc. London, A453 (1997), 2369-2389 (<a href="http://neilsloane.com/doc/Me220.pdf">pdf</a>).
%H Joan Serra-Sagrista, <a href="http://dx.doi.org/10.1016/S0020-0190(00)00119-8">Enumeration of lattice points in l_1 norm</a>, Inf. Proc. Lett. 76 (1-2) (2000) 39-44.
%H <a href="/index/Rec#order_20">Index entries for linear recurrences with constant coefficients</a>, signature (20, -190, 1140, -4845, 15504, -38760, 77520, -125970, 167960, -184756, 167960, -125970, 77520, -38760, 15504, -4845, 1140, -190, 20, -1).
%F Sum_{d=1..20} C(21, d)*C(m/2-1, d-1)*C(20-d+m/2, m/2), where norm m is always even.
%K nonn,easy
%O 0,2
%A Joan Serra-Sagrista (jserra(AT)ccd.uab.es)
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