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 A247820 Numbers n such that sigma(2n-1) is a prime p. 11
 5, 13, 145, 365, 841, 1201, 1741, 2521, 3961, 5101, 7813, 8581, 13945, 14281, 14965, 41761, 42925, 73345, 139921, 229165, 245701, 265721, 276025, 289561, 298765, 341965, 351961, 353641, 367225, 414961, 595141, 601705, 676285, 697381, 711625, 740545, 942565 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Supersequence of A247789. Corresponding values of primes p for a(n) are A008438(a(n)-1)) or A247837. LINKS Martin MÃ¸ller Skarbiniks Pedersen, Table of n, a(n) for n = 1..1000 EXAMPLE Number 13 is in sequence because sigma(2*13-1) = sigma(25) = 31 (prime). PROG (MAGMA) [n: n in [1..10000000] | IsPrime(SumOfDivisors(2*n-1))] (PARI) for(n=1, 10^7, if(isprime(sigma(2*n-1)), print1(n, ", "))) \\ Derek Orr, Sep 25 2014 CROSSREFS Cf. A000203, A008438, A247789, A247837. Sequence in context: A305643 A316919 A099974 * A336097 A187894 A214591 Adjacent sequences:  A247817 A247818 A247819 * A247821 A247822 A247823 KEYWORD nonn AUTHOR Jaroslav Krizek, Sep 24 2014 STATUS approved

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Last modified January 22 23:50 EST 2022. Contains 350504 sequences. (Running on oeis4.)