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 A023194 Numbers n such that sigma(n) (sum of divisors of n) is prime. 62
 2, 4, 9, 16, 25, 64, 289, 729, 1681, 2401, 3481, 4096, 5041, 7921, 10201, 15625, 17161, 27889, 28561, 29929, 65536, 83521, 85849, 146689, 262144, 279841, 458329, 491401, 531441, 552049, 579121, 597529, 683929, 703921, 707281, 734449, 829921, 1190281 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS All numbers except the first are squares. Why? - Zak Seidov, Jun 10 2005 Answer from Gabe Cunningham (gcasey(AT)MIT.EDU): "From the fact that the sigma (the sum-of-divisors function) is multiplicative, we can derive that the sigma(n) is even except when n is a square or twice a square. "If n = 2*(2*k + 1)^2, that is, n is twice an odd square, then sigma(n) = 3*sigma((2*k + 1)^2). If n = 2*(2*k)^2, that is, n is twice an even square, then sigma(n) is only prime if n is a power of 2; otherwise we have sigma(n) = sigma(8*2^m) * sigma(k/2^m) for some positive integer m. "So the only possible candidates for values of n other than squares such that sigma(n) is prime are odd powers of 2. But sigma(2^(2*m + 1)) = 2^(2*m + 2) - 1 = (2^(m + 1) + 1) * (2^(m + 1) - 1), which is only prime when m = 0, that is, when n = 2. So 2 is the only nonsquare n such that sigma(n) is prime." All numbers on this list also have a prime number of divisors. - Howard Berman (howard_berman(AT)hotmail.com), Oct 29 2008 This is because 1 + p + ... + p^k is divisible by 1 + p + ... + p^j if k + 1 is divisible by j + 1. - Robert Israel, Jan 13 2015 From Gabe Cunningham's comment it follows that the alternate Mathematica program provided below is substantially more efficient as it only tests squares. - Harvey P. Dale, Dec 12 2010 Each number of this sequence is a prime power. This follows from the facts that sigma is multiplicative and sigma(n) > 1 for n > 1. Thus, for n > 1, a(n) is of the form a(n) = k^2 where k = p^m, with p prime, so the divisors of a(n) are {1, p, p^2, p^3, ..., (p^m)^2}, and this set is a multiplicative group (modulo q); if q is prime, q = sigma(k^2). Reciprocally, if q is a prime of the form 1 + p + p^2 + ... + p^(2*m), then q = sigma(p^(2*m)) (definition of sigma). - Michel Lagneau, Aug 17 2011, edited by Franklin T. Adams-Watters, Aug 17 2011 The sums of divisors of the even numbers in this sequence are the Mersenne primes, A000668. These even numbers are in A061652. - Hartmut F. W. Hoft, May 04 2015 Numbers n = p^(q - 1), where p is a prime, such that (p^q - 1)/(p - 1) is prime. Then q must be a prime that does not divide p - 1. - Thomas Ordowski, Nov 18 2017 LINKS T. D. Noe and David W. Wilson, Table of n, a(n) for n = 1..10000 MAPLE N:= 10^8: # to get all entries <= N Primes:= select(isprime, [2, seq(2*i+1, i=1..floor((sqrt(N)-1)/2))]): P2:= select(t -> (t > 2 and t < 1 + ilog2(N)), Primes): cands:= {seq(seq([p, q], p=Primes), q=P2)} union {[2, 2]}: f:= proc(pq) local t, j;     t:= pq[1]^(pq[2]-1);     if t <= N and isprime((t*pq[1]-1)/(pq[1]-1)) then t else NULL fi end proc: map(f, cands); # if using Maple 11 or earlier, uncomment the next line # sort(convert(%, list));  # Robert Israel, Jan 13 2015 MATHEMATICA Select[ Range[ 100000 ], PrimeQ[ DivisorSigma[ 1, # ] ]& ] (* David W. Wilson *) Prepend[Select[Range[1100]^2, PrimeQ[DivisorSigma[1, #]]&], 2] (* Harvey P. Dale, Dec 12 2010 *) PROG (PARI) for(x=1, 1000, if(isprime(sigma(x)), print(x))) /* Jorge Coveiro (jorgecoveiro(AT)yahoo.com), Dec 23 2004 */ (PARI) list(lim)=my(v=List([2])); forprime(p=2, sqrtint(lim\=1), if(isprime(p^2+p+1), listput(v, p^2))); forstep(e=4, logint(lim, 2), 2, forprime(p=2, sqrtnint(lim, e), if(isprime((p^(e+1)-1)/(p-1)), listput(v, p^e)))); Set(v) \\ Charles R Greathouse IV, Aug 17 2011; updated Jul 22 2016 (MAGMA) [n: n in [1..2*10^6] | IsPrime(SumOfDivisors(n))]; // Vincenzo Librandi, May 05 2015 (Python) from sympy import isprime, divisor_sigma A023194_list = [2]+[n**2 for n in range(1, 10**3) if isprime(divisor_sigma(n**2))] # Chai Wah Wu, Jul 14 2016 CROSSREFS Cf. A055638 (the square roots of the squares in this sequence). Cf. A023195 (the primes produced by these n). Sequence in context: A110878 A253849 A077137 * A114080 A090676 A261240 Adjacent sequences:  A023191 A023192 A023193 * A023195 A023196 A023197 KEYWORD nonn,easy,nice AUTHOR STATUS approved

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Last modified October 23 16:15 EDT 2018. Contains 316529 sequences. (Running on oeis4.)