

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, (n1)/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



