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A206553 Least prime p > 3 such that 2^n + p*2^floor((n+1)/2) - 1 is prime. 2
5, 5, 13, 7, 5, 5, 7, 7, 11, 13, 13, 11, 5, 11, 37, 11, 5, 23, 13, 47, 89, 13, 19, 19, 11, 7, 19, 23, 17, 13, 19, 43, 29, 79, 61, 17, 191, 43, 337, 53, 29, 17, 13, 13, 29, 11, 19, 11, 11, 13, 43, 163, 29, 13, 7, 53, 23, 97, 31, 29, 41, 83, 79, 23, 191, 97 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Pierre CAMI, Table of n, a(n) for n = 1..10000

EXAMPLE

2^1+5*2^1-1 = 11 prime so a(1) = 5.

2^2+5*2^1-1 = 13 prime so a(2) = 5.

PROG

PFGW64 from Primeform group and SRYPTIFY

command : pfgw64 -f in.txt

in.txt file :

SCRIPT

DIM kk

DIM nn, 0

DIM mm

DIMS tt

OPENFILEOUT myfil, prem.txt

LABEL loopn

SET nn, nn+1

IF nn>10000 THEN END

IF nn%2==0 THEN SET mm, nn/2

IF nn%2==1 THEN SET mm, nn/2+1

SET kk, 2

LABEL loopk

SET kk, kk+1

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

PRP (2^(nn-mm)+p(kk))*2^mm-1, tt

IF ISPRP THEN GOTO a

IF ISPRIME THEN GOTO a

GOTO loopk

LABEL a

WRITE myfil, tt

GOTO loopn

(Haskell)

a206553 n = head [p | p <- drop 2 a000040_list,

a010051 (2^n + p*2^(div (n+1) 2) - 1) == 1]

-- Reinhard Zumkeller, Feb 10 2012

CROSSREFS

Cf. A206554.

Cf. A010051, A000040.

Sequence in context: A338085 A302676 A339947 * A122213 A224067 A049735

Adjacent sequences: A206550 A206551 A206552 * A206554 A206555 A206556

KEYWORD

nonn

AUTHOR

Pierre CAMI, Feb 09 2012

STATUS

approved

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Last modified January 29 23:01 EST 2023. Contains 359939 sequences. (Running on oeis4.)