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A237839 a(n) = |{0 < k <= n: q = |{p <= k*n: p and p + 2 are both prime}| and q + 2 are both prime}|. 3
0, 0, 0, 2, 1, 3, 2, 3, 1, 2, 2, 3, 3, 2, 2, 5, 2, 3, 3, 4, 2, 2, 2, 3, 1, 2, 2, 3, 3, 2, 3, 2, 2, 3, 6, 7, 5, 5, 3, 4, 3, 3, 4, 3, 3, 4, 4, 4, 5, 4, 5, 3, 3, 4, 3, 2, 2, 3, 4, 3, 4, 3, 3, 6, 6, 5, 5, 4, 5, 3, 5, 8, 4, 3, 3, 4, 1, 3, 4, 3 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,4

COMMENTS

Conjecture: a(n) > 0 for all n > 3, and a(n) = 1 only for n = 5, 9, 25, 77, 104.

See also A237838 for a similar conjecture involving Sophie Germain primes.

LINKS

Zhi-Wei Sun, Table of n, a(n) for n = 1..1100

EXAMPLE

a(9) = 1 since {p <= 4*9: p and p + 2 are both prime} = {3, 5, 11, 17, 29} has cardinality 5 and {5, 7} is a twin prime pair.

MATHEMATICA

TQ[n_]:=PrimeQ[n]&&PrimeQ[n+2]

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

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

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

CROSSREFS

Cf. A001359, A006512, A237578, A237769, A237817, A237838.

Sequence in context: A005679 A232927 A275832 * A101608 A102853 A304099

Adjacent sequences:  A237836 A237837 A237838 * A237840 A237841 A237842

KEYWORD

nonn

AUTHOR

Zhi-Wei Sun, Feb 14 2014

STATUS

approved

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Last modified December 2 03:43 EST 2021. Contains 349437 sequences. (Running on oeis4.)