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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

Table of n, a(n) for n=1..8.

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

Cf. A000203, A076274, A247787.

Sequence in context: A245059 A321587 A089815 * A006759 A073513 A074524

Adjacent sequences:  A247785 A247786 A247787 * A247789 A247790 A247791

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 June 26 10:12 EDT 2019. Contains 324375 sequences. (Running on oeis4.)