OFFSET
1,4
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
EXAMPLE
a(4) = 2 because there are two twin prime pairs of the form (4k - 1,4k + 1) with 1 <= k <= 4, namely (3, 5) for k = 1 and (11, 13) for k = 3.
MATHEMATICA
a[n_] := Sum[Boole[PrimeQ[k*n - 1] && PrimeQ[k*n + 1]], {k, 1, n}]; Array[a, 100] (* Amiram Eldar, Dec 25 2019 *)
PROG
(PARI) a(n)=sum(k=1, n, isprime(k*n-1)&&isprime(k*n+1)) \\ Charles R Greathouse IV, Jun 27 2013
CROSSREFS
KEYWORD
nonn
AUTHOR
Irina Gerasimova, Jun 23 2013
EXTENSIONS
a(7), a(20), a(36) corrected by Charles R Greathouse IV, Jun 27 2013
a(54)-a(90) from Charles R Greathouse IV, Jun 27 2013
STATUS
approved