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A247788 Primes p such that sigma(2p-1) = 3*(p-1). 2
3, 17, 131, 193, 449, 13469, 23297, 581150417 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
Primes p such that A247787(p) = A000203(A076274(p)) = 3*(p-1).
If a(9) exists it must be bigger than 10^10.
LINKS
EXAMPLE
Prime 17 is in sequence because sigma(2*17-1) = sigma(33) = 48 = 3*(17-1).
MATHEMATICA
Select[Prime@ Range[10^5], DivisorSigma[1, 2 # - 1] == 3 (# - 1) &] (* Michael De Vlieger, Jan 03 2017 *)
PROG
(Magma) [p: p in PrimesUpTo(20000)| SumOfDivisors(2*p-1) eq 3*p-3]
(PARI) forprime(p=1, 10^7, if(sigma(2*p-1)==3*(p-1), print1(p, ", "))) \\ Derek Orr, Sep 25 2014
CROSSREFS
Sequence in context: A321587 A089815 A328402 * A355598 A006759 A073513
KEYWORD
nonn,more
AUTHOR
Jaroslav Krizek, Sep 24 2014
EXTENSIONS
a(8) from Matthew Campbell, Jan 03 2017
STATUS
approved

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Last modified February 22 10:17 EST 2024. Contains 370250 sequences. (Running on oeis4.)