A162308 Number of twin primes A001097 smaller than the non-twin prime A007510(n).

0, 7, 9, 11, 11, 13, 15, 15, 15, 15, 19, 19, 19, 23, 23, 23, 23, 29, 29, 31, 33, 33, 33, 35, 37, 37, 39, 39, 39, 41, 41, 41, 41, 41, 41, 41, 41, 41, 41, 45, 45, 45, 45, 47, 47, 47, 47, 47, 47, 47, 49, 49, 49, 49, 51, 51, 51, 53, 53, 55, 57, 57, 59, 59, 59, 59, 59, 59, 59

Offset

1,2

Links

Table of n, a(n) for n=1..69.

Example

a(2)=7 counts the numbers 3, 5, 7, 11, 13, 17, 19 below 23=A007510(2).

Maple

isA007510 := proc(n) RETURN(isprime(n) and not isprime(n-2) and not isprime(n+2)) ; end:
isA001097 := proc(n) RETURN(isprime(n) and (isprime(n-2) or isprime(n+2)) ) ; end:
A007510 := proc(n) local a; if n = 1 then 2; else for a from procname(n-1)+1 do if isA007510(a) then RETURN(a) ; fi; od: fi; end:
A162308 := proc(n) local a, k; a := 0 ; for k from 3 to A007510(n)-1 do if isA001097(k) then a := a+1; fi; od; a; end:
seq(A162308(n), n=1..120) ; # R. J. Mathar, Jul 02 2009

Crossrefs

Cf. A000040, A073425.

Sequence in context: A058483 A176326 A283056 * A191883 A108815 A262536

Adjacent sequences: A162305 A162306 A162307 * A162309 A162310 A162311

Keyword

nonn

Author

Juri-Stepan Gerasimov, Jul 01 2009

Extensions

Edited by R. J. Mathar, Jul 02 2009

Status

approved