|
| |
|
|
A111067
|
|
Number of odd primes p < 10^n such that p+2=product of 2 primes (no twin Chen primes).
|
|
0
| | |
|
|
|
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.
|
|
|
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 (pierre-cami(AT)bbox.fr), Oct 08 2005
|
|
|
EXTENSIONS
| a(8) corrected and a(9) computed by Robert G. Wilson v (rgwv(at)rgwv.com), Oct 10 2005
|
| |
|
|
