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A156876 Number of numbers <= n that are safe primes or Sophie Germain primes. 6
0, 1, 2, 2, 3, 3, 4, 4, 4, 4, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 6, 6, 6, 6, 6, 6, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 8, 8, 8, 8, 8, 8, 9, 9, 9, 9, 9, 9, 10, 10, 10, 10, 10, 10, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 12, 12, 12, 12, 12 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

a(n) = A156874(n)+A156875(n)-A156877(n) = A000720(n)-A156878(n).

LINKS

R. Zumkeller, Table of n, a(n) for n = 1..10000

EXAMPLE

a(120)=#{2,3,5,7,11,23,29,41,47,53,59,83,89,107,113}=15.

MATHEMATICA

Accumulate[Table[If[AllTrue[{n, 2n+1}, PrimeQ]||AllTrue[{n, (n-1)/2}, PrimeQ], 1, 0], {n, 100}]] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Sep 24 2019 *)

CROSSREFS

A156658, A005384, A005385, A000720.

Sequence in context: A171896 A094235 A262558 * A137397 A239684 A062571

Adjacent sequences:  A156873 A156874 A156875 * A156877 A156878 A156879

KEYWORD

nonn

AUTHOR

Reinhard Zumkeller, Feb 18 2009

STATUS

approved

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Last modified November 17 23:26 EST 2019. Contains 329242 sequences. (Running on oeis4.)