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%I A171720 #11 Sep 08 2022 08:45:50
%S A171720 29,2729,29683,37747,73133,149837,159667,168269,185371,244147,695477,
%T A171720 880027,891749,910747,1088669,1298309,1371991,1479113,2414717,3531329,
%U A171720 6593729,7452019,9590311,13542811,14527889,15592877,18718939,25650533
%N A171720 Primes p such that (p-1)/2 and (p+1)/2 have same sum of divisors.
%C A171720 If sigma((p-1)/2) is odd then p is of the form n^2+1.
%H A171720 Filip Jevtic, <a href="/A171720/b171720.txt">Table of n, a(n) for n=1,..., 78</a>.
%t A171720 Select[Prime[Range[1700000]],DivisorSigma[1,(#-1)/2] == DivisorSigma[1, (#+1)/2]&] (* _Harvey P. Dale_, Jul 23 2011 *)
%o A171720 (PARI) for(n=2, 10^6, if(sigma((prime(n)-1)/2)==sigma((prime(n)+1)/2), print(prime(n))))
%o A171720 (Magma) [ p: p in PrimesInInterval(3, 30000000) | SumOfDivisors((p-1) div 2) eq SumOfDivisors((p+1) div 2) ];
%K A171720 nonn
%O A171720 1,1
%A A171720 Filip Jevtic (mm07006(AT)alas.math.rs), Dec 16 2009, Dec 19 2009

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