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A344776 a(n) is the least prime p such that there are exactly n pairs of primes (q,r) with p > q > r such that q | r + p and r | q + p. 1
2, 11, 7, 19, 103, 163, 263, 719, 3119, 3779, 6719, 18719, 7559, 67679, 52919, 181439, 138599, 241919, 818999, 262079, 453599, 665279, 1542239, 1713599, 1330559, 4979519, 2741759, 5569199, 7197119, 5745599 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,1

LINKS

Table of n, a(n) for n=0..29.

EXAMPLE

a(3) = 19 because there are 3 pairs (3,2), (7,2), (11,3) with

3 | 2+19, 2 | 3+19, 7 | 2+19, 2 | 7+19, 11 | 3+19, 3 | 11+19.

MAPLE

M:= 10000: N:= ithprime(M): B:= Vector(N):

a:= 1:

do

  a:= nextprime(a);

  if a >= N then break fi;

  b:= a;

  do

    b:= nextprime(b);

    if b >= N then break fi;

    c0:= chrem([-a, -b], [b, a]);

    cs:= select(isprime, [seq(c0+i*a*b, i=ceil((b+2-c0)/(a*b)) .. floor((N-c0)/(a*b)))]);

    B[cs]:= B[cs]+~1;

  od:

od:

V:= Array(0..14):

for i from 2 to N do

  v:= A[i];

  if V[v] = 0 then V[v]:= i fi

od:

convert(V, list);

PROG

(PARI) count_pairs(n) = my(i=0); forprime(q=1, n-1, forprime(r=1, q-1, if((r+n)%q==0 && (q+n)%r==0, i++))); i

a(n) = forprime(p=1, , if(count_pairs(p)==n, return(p))) \\ Felix Fröhlich, May 28 2021

CROSSREFS

Cf. A344788.

Sequence in context: A306537 A170873 A224210 * A290572 A220771 A277055

Adjacent sequences:  A344773 A344774 A344775 * A344777 A344778 A344779

KEYWORD

nonn,more

AUTHOR

J. M. Bergot and Robert Israel, May 28 2021

EXTENSIONS

a(15)-a(17) from Chai Wah Wu, Jun 04 2021

a(18) from Bert Dobbelaere, Jun 07 2021

a(19)-a(21) from Chai Wah Wu, Jun 04 2021

a(22)-a(29) from Bert Dobbelaere, Jun 07 2021

STATUS

approved

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Last modified December 5 23:38 EST 2021. Contains 349558 sequences. (Running on oeis4.)