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A187866 Greatest k such that prime(n)*(prime(n)-k)-1 and prime(n)*(prime(n)-k)+1 are twin primes, k >= 0 and k < prime(n) or -1 if no such k exists. 1
0, 1, -1, 1, -1, -1, 11, 7, 17, 17, -1, -1, 11, 19, 41, -1, 41, -1, 43, 53, -1, -1, 29, 41, -1, 59, 97, 101, 61, 89, -1, 101, 131, 127, 137, 73, 133, 127, 137, 119, 47, 163, 101, 157, 131 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,7

COMMENTS

Conjectures:

1. There are only 11 primes such that k does not exist: 5, 11, 13, 31, 37, 53, 61, 73, 79, 97, 127 (same as A183563).

2. There are only 20 primes such that k(n) = A187563(n): 2, 3, 7, 17, 19, 23, 41, 47, 59, 89, 103, 149, 167, 173, 179, 191, 277, 353, 433, 727.

3. If prime(n) >= 3 there are always at least 2 pairs of twin primes between prime(n) and prime(n)^2.

LINKS

Pierre CAMI, Table of n, a(n) for n = 1..25000

MATHEMATICA

a[n_] := (k=Prime[n]-1; While[p = Prime[n]*(Prime[n]-k)-1; k>=0 && !(PrimeQ[p] && PrimeQ[p + 2]), k--]; k); a /@ Range[45] (* Jean-Fran├žois Alcover, Mar 28 2011 *)

PROG

(PFGW SCRIPTIFY)

SCRIPT

DIM nn, 1

DIM kk

DIMS tt

OPENFILEOUT out, twin

LABEL loopn

SET nn, nn+1

IF nn>25000 THEN END

SET kk, p(nn)

LABEL loopk

SET kk, kk-2

IF kk==-1 THEN GOTO c

SETS tt, %d, %d, %d\,; nn; p(nn); kk

PRP p(nn)*(p(nn)-kk)-1, tt

IF ISPRIME THEN GOTO a

GOTO loopk

LABEL a

PRP p(nn)*(p(nn)-kk)+1, tt

IF ISPRIME THEN GOTO b

GOTO loopk

LABEL b

WRITE out, tt

GOTO loopn

LABEL c

SET kk, -1

SETS tt, %d, %d, %d\,; nn; p(nn); kk

WRITE out, tt

GOTO loopn

CROSSREFS

Cf. A187563.

Sequence in context: A187563 A089487 A166521 * A206419 A250032 A305447

Adjacent sequences:  A187863 A187864 A187865 * A187867 A187868 A187869

KEYWORD

sign

AUTHOR

Pierre CAMI, Mar 14 2011

STATUS

approved

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Last modified May 15 17:50 EDT 2021. Contains 343920 sequences. (Running on oeis4.)