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A342821 a(n) is the least prime p such that p*n+(n-1) and p*n-(n-1) are both prime, or 0 if there is no such p. 2
2, 2, 3, 2, 3, 2, 5, 3, 5, 2, 3, 29, 7, 0, 3, 2, 0, 2, 11, 3, 3, 5, 0, 19, 5, 3, 29, 2, 0, 2, 13, 0, 3, 29, 3, 2, 41, 0, 5, 11, 3, 11, 5, 0, 5, 2, 0, 3, 5, 3, 19, 19, 3, 11, 7, 3, 11, 2, 0, 2, 7, 0, 3, 5, 0, 2, 5, 3, 11, 5, 0, 19, 13, 0, 31, 7, 0, 2, 19, 0, 7, 11, 3, 5, 19, 0, 5, 2, 0, 3, 7, 0, 5 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

If n == 2 (mod 3) then a(n) <= 3.

LINKS

Robert Israel, Table of n, a(n) for n = 1..10000

EXAMPLE

a(7) = 5 because 5*7+6 = 41 and 5*7-6 = 29 are prime, and 5 is the least prime that works.

MAPLE

f:= proc(n) local p, pmax;

  p:= 1:

  if n mod 3 = 2 then pmax := 3 else pmax := infinity fi;

  while p < pmax do

    p:= nextprime(p);

    if isprime(n*p+n-1) and isprime(n*p-n+1) then return p fi;

  od:

  0

end proc:

map(f, [$1..100]);

CROSSREFS

Cf. A342822.

Sequence in context: A105560 A331597 A339502 * A165916 A096013 A072380

Adjacent sequences:  A342818 A342819 A342820 * A342822 A342823 A342824

KEYWORD

nonn

AUTHOR

J. M. Bergot and Robert Israel, Apr 25 2021

STATUS

approved

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Last modified June 24 17:51 EDT 2021. Contains 345419 sequences. (Running on oeis4.)