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A087859 a(n) is the number of twin primes x-1,x+1 such that x=j*(p(n)#)/p(k), where 1 <= j < p(n+1) and 1 <= k <= n and p(k) doesn't divide j. 2
0, 1, 3, 7, 8, 10, 15, 13, 13, 15, 10, 12, 15, 15, 18, 13, 22, 15, 23, 19, 23, 16, 19, 16, 22, 13, 15, 20, 23, 14, 18, 27, 20, 20, 16, 25, 21, 25, 14, 27, 21, 25, 29, 19, 26, 21, 25, 27, 21, 19, 15, 16, 32, 17, 19, 19, 21, 17, 22, 23, 29, 24, 29, 29, 18, 25, 25 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

p(n) is the n-th prime; # denotes primorial (A002110).

a(n) seems to grow like 4*log(p(n)).

LINKS

Table of n, a(n) for n=1..67.

EXAMPLE

a(3) = 3 because for (j,k) = (1,3),(2,3),(3,3), j*(5#)/p(k)+-1 are primes.

PROG

(PARI) a(n) = {my(p=vector(n, i, prime(i)), x, y=prod(i=1, n, p[i])); sum(j=1, prime(n+1)-1, sum(k=1, n, j%p[k]>0 && ispseudoprime(x=j*y/p[k]-1) && ispseudoprime(x+2))); } \\ Jinyuan Wang, Mar 20 2020

CROSSREFS

Cf. A002110, A087941, A088968.

Sequence in context: A019270 A047357 A003607 * A300082 A050010 A316493

Adjacent sequences:  A087856 A087857 A087858 * A087860 A087861 A087862

KEYWORD

nonn

AUTHOR

Pierre CAMI, Oct 25 2003

EXTENSIONS

Edited by Don Reble, Nov 16 2005

More terms from Jinyuan Wang, Mar 20 2020

STATUS

approved

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Last modified April 23 13:25 EDT 2021. Contains 343204 sequences. (Running on oeis4.)