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A210733 Least k > n-1 such that 4^n + 2^k - 1 is a prime number. 2
1, 2, 3, 4, 6, 6, 12, 9, 10, 10, 27, 13, 15, 22, 16, 16, 18, 19, 39, 22, 22, 43, 24, 24, 48, 26, 28, 43, 32, 38, 75, 33, 36, 58, 35, 57, 55, 47, 48, 52, 54, 72, 52, 51, 72, 54, 72, 53, 64, 62, 52, 52, 63, 60, 55, 60, 316, 70, 63, 68, 64, 96, 66, 115, 66, 92 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

As n increases k is on average 1.43*n.

LINKS

Pierre CAMI, Table of n, a(n) for n = 1..3867

EXAMPLE

4^1+2^1-1=5 prime so a(1)=1.

4^2+2^2-1=19 prime so a(2)=2.

4^3+2^3-1=71 prime so a(3)=3.

MATHEMATICA

Table[k = n; While[! PrimeQ[4^n + 2^k - 1], k++];  k, {n, 100}] (* T. D. Noe, May 16 2012 *)

PROG

(PFGW64 and SCRIPTIFY)

Command : PFGW64 -f in.txt

in.txt file :

SCRIPT

DIM nn, 0

DIM kk

DIMS tt

OPENFILEOUT myfile, a(n).txt

LABEL loopn

SET nn, nn+1

SET kk, nn-1

LABEL loopk

SET kk, kk+1

SETS tt, %d, %d\,; nn; kk

PRP 4^nn+2^kk-1, tt

IF ISPRP THEN GOTO a

IF ISPRIME THEN GOTO a

GOTO loopk

LABEL a

WRITE myfile, tt

GOTO loopn

CROSSREFS

Sequence in context: A317491 A028335 A007464 * A265564 A064764 A123131

Adjacent sequences:  A210730 A210731 A210732 * A210734 A210735 A210736

KEYWORD

nonn

AUTHOR

Pierre CAMI, May 10 2012

EXTENSIONS

Missing a(2103) inserted into b-file by Andrew Howroyd, Feb 24 2018

STATUS

approved

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Last modified November 20 05:07 EST 2019. Contains 329323 sequences. (Running on oeis4.)