A189977 Primes p such that d(p+1) = 2*d(p-1), where d(k) counts the divisors of k.

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

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: A053299 A356828 A220239 * A130547 A352355 A200846

Adjacent sequences: A189974 A189975 A189976 * A189978 A189979 A189980

Keyword

nonn

Author

Ctibor O. Zizka, May 03 2011

Status

approved