mersenneforum.org Are Mersenne numbers really square?
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 2018-02-28, 12:10 #1 JM Montolio A   Feb 2018 25·3 Posts Are Mersenne numbers really square? What do you think about it ? JMMA
 2018-02-28, 15:56 #2 GP2     Sep 2003 A1B16 Posts You mean Mersenne numbers with prime exponents. Nobody knows for sure. If you ever find one that's non-square-free, then you automatically also find the third known Wieferich prime, and that would be a pretty big deal.
 2018-02-28, 18:05 #3 CRGreathouse     Aug 2006 176116 Posts It's an interesting question. I seem to recall a weak consensus that there probably are Mersenne numbers with prime exponents which are not squarefree, but they may be extremely large. Does anyone here have an opinion?
2018-02-28, 18:39   #4
science_man_88

"Forget I exist"
Jul 2009
Dumbassville

838410 Posts

Quote:
 Originally Posted by CRGreathouse It's an interesting question. I seem to recall a weak consensus that there probably are Mersenne numbers with prime exponents which are not squarefree, but they may be extremely large. Does anyone here have an opinion?
(Mod(1,3)*j*p^2+Mod(2,3)*p)*k^2+(Mod(1,3)*j*p+Mod(2,3))*k+Mod(1,3)*j roots mod 3 is what it comes down to partially.

2018-02-28, 20:17   #5
Dr Sardonicus

Feb 2017
Nowhere

24×32×31 Posts

Quote:
 Originally Posted by CRGreathouse It's an interesting question. I seem to recall a weak consensus that there probably are Mersenne numbers with prime exponents which are not squarefree, but they may be extremely large. Does anyone here have an opinion?
Many years ago, I heard that whether there are infinitely many Mersenne numbers that are squarefree was an open question. I may be misremembering, but if that actually is unknown, it would illustrate how difficult this sort of question may be.

Any non-squarefree Mersenne numbers with prime exponent certainly would be "extremely large," at least by my standards. As already pointed out, the exponent would be the next known Wieferich prime. The best known lower bound I could find in a quick online search was

A Wieferich Prime Search up to 6.7 × 1015.

From the abstract,

Quote:
 This paper describes a new search algorithm for Wieferich primes using double-precision Montgomery arithmetic and a memoryless sieve, which runs significantly faster than previously published algorithms, allowing us to report that there are no other Wieferich primes p < 6.7 × 1015.

2018-02-28, 20:21   #6
science_man_88

"Forget I exist"
Jul 2009
Dumbassville

26·131 Posts

Quote:
 Originally Posted by JM Montolio A JMMA
Do you know about clock math ? If not you may not understand responses.

2018-02-28, 21:01   #7
CRGreathouse

Aug 2006

10111011000012 Posts

Quote:
 Originally Posted by Dr Sardonicus Many years ago, I heard that whether there are infinitely many Mersenne numbers that are squarefree was an open question. I may be misremembering, but if that actually is unknown, it would illustrate how difficult this sort of question may be.
I could believe it! It's like abc implying infinitely many non-Wieferich primes.

Quote:
 Originally Posted by Dr Sardonicus Any non-squarefree Mersenne numbers with prime exponent certainly would be "extremely large," at least by my standards. As already pointed out, the exponent would be the next known Wieferich prime. The best known lower bound I could find in a quick online search was A Wieferich Prime Search up to 6.7 × 1015.
PrimeGrid got to 6.071319 × 1017 before they suspended their search last May.

2018-02-28, 21:30   #8
GP2

Sep 2003

13×199 Posts

Quote:
 Originally Posted by Dr Sardonicus As already pointed out, the exponent would be the next known Wieferich prime.
It's not the exponent that would be the next Wieferich prime, but the non-square-free factor itself.

There is good reason to believe that all factors of size 64 bits or less of Mersenne exponents under 1 billion have already been found, either by TF or by user TJAOI, as well a considerable percentage of 65 bit factors. So all new Mersenne factors being discovered are in the 1020 range, which is beyond the range of the various exhaustive searches that were done by Dorais and Klyve, and I think also by PrimeGrid at one point.

So theoretically, every new factor we find has some infinitesimal chance of being the next Wieferich prime.

2018-02-28, 21:56   #9
Prime95
P90 years forever!

Aug 2002
Yeehaw, FL

1CEF16 Posts

Quote:
 Originally Posted by GP2 So theoretically, every new factor we find has some infinitesimal chance of being the next Wieferich prime.
The prim95 client and the PrimeNet server do not check for this. Perhaps mersenne.ca does (or could be upgraded to) check.

2018-02-28, 23:03   #10
GP2

Sep 2003

13×199 Posts

Quote:
 Originally Posted by Prime95 The prim95 client and the PrimeNet server do not check for this. Perhaps mersenne.ca does (or could be upgraded to) check.
But it would be trivially easy to implement this. It's a small variation on the checking that's already done to verify a factor.

If f is a factor of 2p−1, then 2p ≡ 1 (mod f).

In other words, that the modular exponentiation of (2,p,f) equals 1.

Modular exponentiation is available in various languages like PHP or Python, and also in the GMP library in the mpz_powm functions.

To check for Weiferich primes, you do the exact same thing, except using modulo f squared.

If the modular exponentiation of (2,p,f2) ever equals 1, it's time to prepare a press release.

From time to time I've run a check over the entire database of factors, it can be done quite quickly. But it would be better to do it each time a factor is reported.

 2018-03-01, 13:07 #11 a1call     "Rashid Naimi" Oct 2015 Remote to Here/There 111110101102 Posts I think it is not impossible. So looking long enough should yield hits. The Mersenne-Pseudoorimes are a subset of the more general form: (k+1)^p-k^p For positive integer k and prime p As such 3^11-2^11 is divisible by 23^2

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