

A224979


Number of primes of the form pq+1 where q is any prime < p = prime(n).


2



0, 1, 1, 2, 2, 3, 4, 4, 6, 6, 4, 3, 8, 6, 10, 10, 12, 5, 4, 12, 9, 8, 16, 18, 6, 16, 10, 16, 12, 20, 6, 18, 16, 14, 24, 8, 9, 10, 26, 22, 28, 12, 22, 13, 26, 16, 12, 14, 24, 18, 30, 36, 16, 32, 28, 32, 38, 14, 13, 32, 16, 38, 16, 34, 17, 30, 12, 18, 32, 26
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OFFSET

1,4


LINKS

Harvey P. Dale, Table of n, a(n) for n = 1..1000


EXAMPLE

For n=5, p=11, there are a(5)=2 solutions: 115+1=7 and 117+1=5.


MATHEMATICA

Table[p = Prime[n]; c = 0; i = 1; While[i < n, If[PrimeQ[p  Prime[i] + 1], c = c + 1]; i++]; c, {n, 70}]
Table[Count[Prime[n]Prime[Range[n1]]+1, _?PrimeQ], {n, 70}] (* Harvey P. Dale, Jan 08 2015 *)


PROG

(PARI) a(n)=my(p=prime(n), s); forprime(q=2, p1, s+=isprime(pq+1)); s \\ Charles R Greathouse IV, Apr 22 2013


CROSSREFS

Cf. A224748, A224908.
Sequence in context: A206922 A276775 A271169 * A211317 A126246 A173633
Adjacent sequences: A224976 A224977 A224978 * A224980 A224981 A224982


KEYWORD

nonn


AUTHOR

Jayanta Basu, Apr 22 2013


STATUS

approved



