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A239638 Numbers n such that the semiprime 2^n-1 is divisible by 2n+1. 1
11, 23, 83, 131, 3359, 130439 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

All terms are primes == 5 modulo 6 (A005384 Sophie Germain primes).

a(7) > 250000. - Giovanni Resta, Mar 23 2014

LINKS

Table of n, a(n) for n=1..6.

EXAMPLE

n = 11, 2^n -1 = 2047 = 23*89,

n = 23, 8388607 = 47*178481,

n = 131, 2722258935367507707706996859454145691647 =  263*10350794431055162386718619237468234569.

MATHEMATICA

Select[Range[4000], PrimeQ[2*# + 1] && PowerMod[2, #, 2*# + 1] == 1 &&

PrimeQ[(2^# - 1)/(2*# + 1)] &] (* Giovanni Resta, Mar 23 2014 *)

PROG

(PARI) is(n)=n%6==5 && Mod(2, 2*n+1)^n==1 && isprime(2*n+1) && ispseudoprime((2^n-1)/(2*n+1)) \\ Charles R Greathouse IV, Aug 25 2016

(Python)

from sympy import isprime, nextprime

A239638_list, p = [], 5

while p < 10**6:

    if (p % 6) == 5:

        n = (p-1)//2

        if pow(2, n, p) == 1 and isprime((2**n-1)//p):

            A239638_list.append(n)

    p = nextprime(p) # Chai Wah Wu, Jun 05 2019

CROSSREFS

Cf. A000043, A001348, A046051, A005384, A005420, A085724, A049479.

Sequence in context: A060160 A241973 A158021 * A050767 A145918 A077707

Adjacent sequences:  A239635 A239636 A239637 * A239639 A239640 A239641

KEYWORD

nonn,more

AUTHOR

Zak Seidov, Mar 23 2014

EXTENSIONS

a(5)-a(6) from Giovanni Resta, Mar 23 2014

STATUS

approved

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Last modified August 20 09:56 EDT 2019. Contains 326143 sequences. (Running on oeis4.)