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A242132 Smallest k such that (2*k*3^n+1)*2*k*3^n-1 is prime, with k not divisible by 3. 4
1, 7, 1, 4, 1, 8, 11, 5, 20, 1, 16, 10, 1, 8, 2, 5, 5, 5, 2, 50, 20, 128, 11, 13, 23, 5, 52, 2, 20, 38, 1, 5, 11, 1, 14, 22, 10, 31, 2, 35, 8, 107, 112, 103, 80, 22, 40, 104, 20, 1, 29, 40, 1, 61, 77, 8, 41, 62, 1, 5, 46, 20, 35, 29, 68, 23, 85, 49, 58, 23 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Conjectures: the ratio a(n)/n is always <10 and sum(a(n)/n)/N for n=1 to N tends to 1 as N tends to infinity.

LINKS

Pierre CAMI, Table of n, a(n) for n = 1..4000

PROG

(PFGW & SCRIPT)

SCRIPT

DIM n, 0

DIM i

DIM pp

DIMS t

OPENFILEOUT myf, a(n).txt

LABEL loop1

SET n, n+1

SET i, 0

LABEL loop2

SET i, i+1

SETS t, %d, %d\,; n; i

SET pp, (2*i*3^n+1)*2*i*3^n-1

PRP pp, t

IF ISPRP THEN GOTO a

GOTO loop2

LABEL a

WRITE myf, t

GOTO loop1

(PARI) a(n) = {k = 1; while (! isprime((2*k*3^n+1)*2*k*3^n-1) || !(k % 3), k++); k; } \\ Michel Marcus, May 05 2014

CROSSREFS

Cf. A242085, A242131, A242133.

Sequence in context: A176439 A048834 A010504 * A011450 A231015 A183031

Adjacent sequences:  A242129 A242130 A242131 * A242133 A242134 A242135

KEYWORD

nonn

AUTHOR

Pierre CAMI, May 05 2014

STATUS

approved

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Last modified June 20 11:30 EDT 2021. Contains 345164 sequences. (Running on oeis4.)