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A156875 Number of safe primes <= n. 8
0, 0, 0, 0, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,7

COMMENTS

a(n) = SUM(A156659(k): 1<=k<=n);

a(n) = A156874(floor((n-1)\2)).

LINKS

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

Wikipedia, Safe prime

EXAMPLE

a(120) = #{5, 7, 11, 23, 47, 59, 83, 107} = 8.

CROSSREFS

A005385, A156874, A156876, A156877, A156878, A000720.

Sequence in context: A084558 A320065 A163291 * A066339 A052375 A212625

Adjacent sequences:  A156872 A156873 A156874 * A156876 A156877 A156878

KEYWORD

nonn

AUTHOR

Reinhard Zumkeller, Feb 18 2009

STATUS

approved

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Last modified November 11 22:31 EST 2019. Contains 329046 sequences. (Running on oeis4.)