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%I #9 Mar 20 2016 12:57:36
%S 13,61,157,181,193,337,385,397,541,673,733,769,877,1153,1201,1213,
%T 1453,1873,1933,2017,2029,2557,2593,2797,3217,3313,3517,4177,4273,
%U 4561,4621,4657,5101,5233,5437,5581,5641,6337,6637,6781,7057,7213,7393,7481,7537,7561,7933,8221,8317
%N Odd integers n such that the sum of the Pell numbers A000129(0) + ... + A000129(n-1) is divisible by n*(n+1)/2.
%C Sequence contains the prime numbers most of the time. Nonprime terms of this sequence are 385, 111361, 111841, 155041, 186961 ...
%e 13 is a term because (0 + 1 + 2 + 5 + 12 + 29 + 70 + 169 + 408 + 985 + 2378 + 5741 + 13860) / (1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 + 11 + 12 + 13) = 260.
%o (PARI) a048739(n) = local(w=quadgen(8)); -1/2+(3/4+1/2*w)*(1+w)^n+(3/4-1/2*w)*(1-w)^n;
%o for(n=1, 1e4, if(a048739(n-1) % ((n+1)*(n+2)/2) == 0 && (n+1) % 2 == 1, print1((n+1), ", ")));
%Y Cf. A000129, A048739, A270342.
%K nonn
%O 1,1
%A _Altug Alkan_, Mar 17 2016
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