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A171720
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Primes p such that (p-1)/2 and (p+1)/2 have same sum of divisors.
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3
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29, 2729, 29683, 37747, 73133, 149837, 159667, 168269, 185371, 244147, 695477, 880027, 891749, 910747, 1088669, 1298309, 1371991, 1479113, 2414717, 3531329, 6593729, 7452019, 9590311, 13542811, 14527889, 15592877, 18718939, 25650533
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OFFSET
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1,1
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COMMENTS
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If sigma((p-1)/2) is odd then p is of the form n^2+1.
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LINKS
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MATHEMATICA
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Select[Prime[Range[1700000]], DivisorSigma[1, (#-1)/2] == DivisorSigma[1, (#+1)/2]&] (* Harvey P. Dale, Jul 23 2011 *)
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PROG
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(PARI) for(n=2, 10^6, if(sigma((prime(n)-1)/2)==sigma((prime(n)+1)/2), print(prime(n))))
(Magma) [ p: p in PrimesInInterval(3, 30000000) | SumOfDivisors((p-1) div 2) eq SumOfDivisors((p+1) div 2) ];
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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Filip Jevtic (mm07006(AT)alas.math.rs), Dec 16 2009, Dec 19 2009
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STATUS
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approved
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