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A189977 Primes p such that d(p+1) = 2*d(p-1), where d(k) counts the divisors of k. 2
2, 23, 149, 293, 311, 439, 557, 569, 743, 773, 857, 1031, 1151, 1493, 1607, 1663, 1709, 1733, 1879, 1913, 2069, 2141, 2423, 2711, 2719, 2729, 2789, 2969, 3191, 3209, 3559, 3607, 3767, 3821, 3833, 3847, 3929, 3967, 4019, 4073, 4229, 4339, 4451, 4517, 4549 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

G. C. Greubel, Table of n, a(n) for n = 1..10000

MATHEMATICA

Select[Prime[Range[PrimePi[5059]]], 2*DivisorSigma[0, # - 1] == DivisorSigma[0, # + 1] &] (* T. D. Noe, May 03 2011 *)

PROG

(PARI) isok(p) = isprime(p) && (numdiv(p+1) == 2*numdiv(p-1)); \\ Michel Marcus, Jan 13 2018

CROSSREFS

Cf. A000005, A008328, A008329.

Sequence in context: A235594 A053299 A220239 * A130547 A200846 A198851

Adjacent sequences:  A189974 A189975 A189976 * A189978 A189979 A189980

KEYWORD

nonn

AUTHOR

Ctibor O. Zizka, May 03 2011

STATUS

approved

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Last modified November 20 10:09 EST 2019. Contains 329334 sequences. (Running on oeis4.)