double post but...i can't resist it...
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[url]http://mersenne-aries.sili.net/exponent.php?exponentdetails=397[/url]
[url]http://www.wolframalpha.com/input/?i=primality+of+%282%5E397-1%29%2F%282383*6353*50023*53993*202471*5877983*814132872808522587940886856743*1234904213576000272542841146073%29[/url]

Lol.
@James: Is WolframAlpha correct, in that the cofactor is prime? If so, why is it not listed? I know PrimeNet does it to save space, but...

Nvm the first question, just found your factoring utility.

See also here for factored [url=http://factorization.ath.cx/index.php?query=M397]M397[/url].

[QUOTE=Dubslow;289554][URL]http://mersenne-aries.sili.net/exponent.php?exponentdetails=397[/URL]
[URL]http://www.wolframalpha.com/input/?i=primality+of+%282%5E397-1%29%2F%282383*6353*50023*53993*202471*5877983*814132872808522587940886856743*1234904213576000272542841146073%29[/URL]

Lol.
@James: Is WolframAlpha correct, in that the cofactor is prime? If so, why is it not listed?[/quote]It's been a common practice not to list the largest prime factor in such tables for many decades, starting in the pencil-and-paper days. Those who want to know its value can perform the appropriate divisions to get it.

Yes, but they also have to verify whether or not it is prime. These days that is trivial, but in the old days surely they must have marked somehow "Fully factored, last factor not listed."

[QUOTE=Dubslow;289607]Yes, but they also have to verify whether or not it is prime. These days that is trivial, but in the old days surely they must have marked somehow "Fully factored, last factor not listed."[/QUOTE]Did you notice that I wrote, "largest [i]prime[/i] factor"?

[QUOTE=cheesehead;289673]Did you notice that I wrote, "largest [i]prime[/i] factor"?[/QUOTE]

Yes, but the databases don't say that, I have to go check that myself. If no one marks that the exponent is in fact FF, then no one will know that it [i]is[/i] prime. It's easy enough to find the cofactor, yes, but nowhere in the databases are the cofactors marked as known primes.

[QUOTE=Dubslow;289675]Yes, but the databases don't say that[/QUOTE]The [I]GIMPS/PrimeNet[/I] database doesn't, because GIMPS's goals do not include completely factoring composite Mersenne numbers. It just records all factors that are reported to PrimeNet.

Which other databases have you checked?

mersenne-aries, which you can check was the original DB referenced. It does not report any Mersennes as fully factored. In the example I posted, the last cofactor is prime, but is not listed, and the page shows only "74% factored" or however it is.

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