Sophie Germain prime and Mersenne number 2^p1
How can I prove that if p=3 (mod 4) is a Sophie Germain prime then the Mersenne number 2^p1 is composite?
Thanks in advance. 
[QUOTE=Tomazio;606085]How can I prove that if p=3 (mod 4) is a Sophie Germain prime then the Mersenne number 2^p1 is composite?
Thanks in advance.[/QUOTE] See the following manuscript: Édouard Lucas, Théorie des Fonctions Numériques Simplement Périodiques, American Journal of Mathematics, Vol. 1, No. 4 (1878), pp. 184240 and 289321 (in French). Available: <[url]http://edouardlucas.free.fr/oeuvres/Theorie_des_fonctions_simplement_periodiques.pdf[/url]>. 
[QUOTE=Tomazio;606085]How can I prove that if p=3 (mod 4) is a Sophie Germain prime then the Mersenne number 2^p1 is composite?
Thanks in advance.[/QUOTE] Because for all such primes p, 2^p1 is divisible by 2*p+1, since 2*p+1 is == 7 mod 8, thus 2 is a [URL="https://en.wikipedia.org/wiki/Quadratic_residue"]quadratic residue[/URL] mod 2*p+1, and znorder(Mod(2,2*p+1)) must divide ((2*p+1)1)/2 = p, thus znorder(Mod(2,2*p+1)) is either 1 or p, but cannot be 1 since there is no prime q such that znorder(Mod(2,q)) = 1 (or q must divide 21 = 1, which is impossible), thus znorder(Mod(2,2*p+1)) can only be p, and hence 2*p+1 divides 2^p1, thus 2^p1 is composite. Similarly, if p == 1 mod 4 is a Sophie Germain prime and p > 5, then the Wagstaff number (2^p+1)/3 is composite. Here is an exercise for you: prove that if p is a Sophie Germain prime other than 2, 3, and 5, then the dozenal repunit (12^p1)/11 is composite. 
[QUOTE=Tomazio;606085]How can I prove that if p=3 (mod 4) is a Sophie Germain prime then the Mersenne number 2^p1 is composite?
Thanks in advance.[/QUOTE] For the first known proof, see the following manuscript: Joseph Louis de Lagrange, Recherches d'arithmétique (1775), pp. 695795 (in French). Available: <[url]https://gallica.bnf.fr/ark:/12148/bpt6k229222d/f696#[/url]>. See [COLOR="Red"]Lemme III[/COLOR] in page 778 and also [COLOR="red"]49. Scolie I[/COLOR] in page 794. 
See also the web page on "Euler and Lagrange on Mersenne Divisors" by Chris K. Caldwell at <[url]https://primes.utm.edu/notes/proofs/MerDiv2.html[/url]>.

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