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 A281882 Numbers n such that the number of divisors of sum of divisors of n is prime. 1
 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Numbers n such that A062068(n) = A000005(A000203(n)) = tau(sigma(n)) is a prime. 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 LINKS Charles R Greathouse IV, Table of n, a(n) for n = 1..1500 EXAMPLE 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, ... MAPLE with(numtheory): A281882:=n->`if`(isprime(tau(sigma(n))), n, NULL): seq(A281882(n), n=1..10^5); # Wesley Ivan Hurt, Feb 01 2017 MATHEMATICA Select[Range[10^6], PrimeQ@ DivisorSigma[0, DivisorSigma[1, #]] &] (* Michael De Vlieger, Feb 04 2017 *) PROG (MAGMA) [n: n in[1..10^7] | IsPrime(NumberOfDivisors(SumOfDivisors(n)))] (PARI) isok(n) = isprime(numdiv(sigma(n))); \\ Michel Marcus, Feb 01 2017 CROSSREFS Supersequence of A023194. Cf. A000005, A000203, A009087, A062068. Sequence in context: A119721 A098969 A235401 * A325436 A346777 A338585 Adjacent sequences:  A281879 A281880 A281881 * A281883 A281884 A281885 KEYWORD nonn AUTHOR Jaroslav Krizek, Feb 01 2017 STATUS approved

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Last modified January 23 11:42 EST 2022. Contains 350512 sequences. (Running on oeis4.)