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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, 406583 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS All terms are primes == 5 modulo 6 (A005384 Sophie Germain primes). a(8) >= 500000. - Max Alekseyev, May 28 2022 LINKS 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 a(7) from Eric Chen, added by Max Alekseyev, May 21 2022 STATUS approved

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Last modified August 7 10:19 EDT 2022. Contains 355985 sequences. (Running on oeis4.)