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A237838 a(n) = |{0 < k <= n: the number of Sophie Germain primes among 1, ..., k*n is a Sophie Germain prime}|. 2
0, 1, 3, 2, 3, 2, 2, 2, 2, 1, 1, 2, 3, 2, 1, 1, 1, 1, 1, 1, 2, 1, 3, 3, 2, 3, 2, 1, 2, 1, 2, 1, 2, 3, 2, 2, 1, 3, 3, 4, 4, 4, 3, 4, 1, 1, 3, 3, 2, 3, 1, 1, 2, 2, 4, 1, 4, 3, 5, 4, 5, 4, 3, 4, 3, 4, 3, 2, 1, 4, 3, 4, 6, 1, 3, 3, 3, 4, 2, 3 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

Conjecture: a(n) > 0 for all n > 1.

See also A237839 for a similar conjecture involving twin primes.

LINKS

Zhi-Wei Sun, Table of n, a(n) for n = 1..1000

EXAMPLE

a(20) = 1 since 11 is a Sophie Germain prime, and there are exactly 11 Sophie Germain primes among 1, ..., 6*20 (namely, they are 2, 3, 5, 11, 23, 29, 41, 53, 83, 89, 113).

MATHEMATICA

SG[n_]:=PrimeQ[n]&&PrimeQ[2n+1]

sg[n_]:=Sum[If[PrimeQ[2*Prime[k]+1], 1, 0], {k, 1, PrimePi[n]}]

a[n_]:=Sum[If[SG[sg[k*n]], 1, 0], {k, 1, n}]

Table[a[n], {n, 1, 80}]

CROSSREFS

Cf. A005384, A237578, A237768, A237815, A237839.

Sequence in context: A099891 A241173 A096835 * A262880 A249355 A064654

Adjacent sequences:  A237835 A237836 A237837 * A237839 A237840 A237841

KEYWORD

nonn

AUTHOR

Zhi-Wei Sun, Feb 14 2014

STATUS

approved

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Last modified July 15 14:25 EDT 2019. Contains 325031 sequences. (Running on oeis4.)