A111067 Number of odd primes p < 10^n such that p+2=product of 2 primes (no twin Chen primes).

1, 11, 79, 427, 3009, 21779, 166649, 1322266, 10752066

Offset

1,2

Comments

A006880(n)=number of primes < 10^n, A007508(n)=number of twin primes < 10^n. Let F(n) = A006880(n)/A007508(n). For n >3, we find that F(n) is ~ 0.762373*log(10^n) - 0.968855.

Let FF(n) = A006880(n)/a(n). For n>3, we find that FF(n) is ~ 0.163128*log(10^n) + 1.349255. a(n)/A007508(n) is then ~ 0.762373*log((10^n) - 0.968855 / ( 0.163128*log(10^n) + 1.349255, as n tends to infinity a(n)/ A007508(n) needs to tend to 0.762373 / 0.163128 = 4.673465.

Links

Table of n, a(n) for n=1..9.

Example

7 is the only prime <10 with 7+2=3*3=product of 2 odd primes so a(1)=1.

Crossrefs

Cf. A006880, A007508.

Sequence in context: A140542 A101983 A139953 * A172067 A026841 A026848

Adjacent sequences: A111064 A111065 A111066 * A111068 A111069 A111070

Keyword

nonn

Author

Pierre CAMI, Oct 08 2005

Extensions

a(8) corrected and a(9) computed by Robert G. Wilson v, Oct 10 2005

Status

approved