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 A065404 Squares of composite numbers k such that sigma(k) (sum of divisors of k, A000203) is a prime. 6
 16, 64, 729, 2401, 4096, 15625, 28561, 65536, 83521, 262144, 279841, 531441, 707281, 3418801, 4826809, 9765625, 24137569, 28398241, 38950081, 47458321, 244140625, 260144641, 887503681, 1073741824, 1387488001, 2655237841 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Amiram Eldar, Table of n, a(n) for n = 1..500 (terms 1..100 from Harry J. Smith) FORMULA sigma(a(n)) = A065403(n). EXAMPLE 46 cases below 10^12; for M a Mersenne prime, (M+1)/2 is here: M=8191, 4096=(M+1)/2. PROG (PARI) { n=0; for (m=1, 10^9, if (isprime(m), next); x=sigma(m^2); if (isprime(x), write("b065404.txt", n++, " ", m^2); if (n==100, return)) ) } \\ Harry J. Smith, Oct 18 2009 CROSSREFS Cf. A062700, A000203 (sigma), A065403, A065405, A053182, A053183, A028982. Sequence in context: A141840 A203281 A255576 * A327496 A330824 A189806 Adjacent sequences: A065401 A065402 A065403 * A065405 A065406 A065407 KEYWORD nonn AUTHOR Labos Elemer, Nov 06 2001 STATUS approved

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Last modified August 11 23:26 EDT 2024. Contains 375080 sequences. (Running on oeis4.)