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Half the number of length n integer sequences with sum zero and sum of squares 648.
1

%I #4 Mar 31 2012 12:35:25

%S 1,3,54,1830,68175,2604105,72741487,1575681348,32584952460,

%T 663445574880,13119864945495,250743317040879,4601891725568592,

%U 80821202263252920,1358608769903073690,21985893765384549032,344851090651019364715

%N Half the number of length n integer sequences with sum zero and sum of squares 648.

%H R. H. Hardin, <a href="/A157550/b157550.txt">Table of n, a(n) for n=2..50</a>

%F [cache enabling] count(n,s,ss)->count(n,t,tt) where t=s mod n, q=(t-s)/n, tt=ss+2*q*s+n*q^2; count(n,t,tt)=sum[i^2<=tt]{ count(n-1,t-i,tt-i^2) }. a(n)=count(n,0,648)/2.

%K nonn

%O 2,2

%A _R. H. Hardin_ Mar 02 2009