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A281882
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Numbers n such that the number of divisors of sum of divisors of n is prime.
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1
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2, 3, 4, 9, 16, 25, 64, 81, 289, 400, 651, 729, 889, 1681, 2401, 2667, 3481, 3937, 4096, 5041, 7921, 10201, 15625, 17161, 27889, 28561, 29929, 57337, 65536, 83521, 85849, 146689, 172011, 253921, 262144, 279841, 458329, 491401, 531441, 552049, 579121, 597529
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OFFSET
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1,1
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COMMENTS
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Conjecture: 2 and 3 are the only prime numbers in the sequence.
Proof: If p is an odd prime, then sigma(p) = p+1 is even and hence the number of divisors of sigma(p) is composite unless p+1 is a power of 2, in which case let p+1 = 2^k which has k+1 divisors. But then p = 2^k - 1 is divisible by 2^(k/2) - 1, which is greater than 1 for p > 3. - Charles R Greathouse IV, Feb 01 2017
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LINKS
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EXAMPLE
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81 is a term because tau(sigma(81)) = tau(121) = 3 (prime).
For n>=1; tau(a(n)) = 2, 3, 2, 2, 2, 2, 2, 3, 2, 3, 11, 2, 11, 2, 2, 13, 2, 13, 2, 2, 2, 2, 2, 2, 2, 2, 2, 17, 2, 2, 2, ...
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MAPLE
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MATHEMATICA
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Select[Range[10^6], PrimeQ@ DivisorSigma[0, DivisorSigma[1, #]] &] (* Michael De Vlieger, Feb 04 2017 *)
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PROG
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(Magma) [n: n in[1..10^7] | IsPrime(NumberOfDivisors(SumOfDivisors(n)))]
(PARI) isok(n) = isprime(numdiv(sigma(n))); \\ Michel Marcus, Feb 01 2017
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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