[QUOTE=davar55;306967]Just learned that M1061 has been factored, as P1 x P2 (two prime factors).

Since 1061 and 1901 are prime upside-down partners ,
I ran factorbd on 2^1901-1 and found a P501 as largest factor.

I think it computed rather than did a lookup.

Is this P501 previously unrecorded? Just thought I'd ask.[/QUOTE]

That 501 is not prime. Black numbers are prime, red are unknown, red/brown are probably prime, and blue is known composite. The [URL="http://factordb.com/index.php?id=1100000000216805652"]C501[/URL] has been in the database since before March 2011. (Follow the link and click on more info.) Edit: If you click more info on M1901 itself, it has the same entry-creation date, which is probably a date the DB underwent maintenance or something. GIMPS has probably gotten the ECM up to t60 or beyond, so there's little hope of factoring it anytime in the near future.

[QUOTE=Dubslow;306970]That 501 is not prime. Black numbers are prime, red are unknown, red/brown are probably prime, and blue is known composite. The [URL="http://factordb.com/index.php?id=1100000000216805652"]C501[/URL] has been in the database since before March 2011. (Follow the link and click on more info.) Edit: If you click more info on M1901 itself, it has the same entry-creation date, which is probably a date the DB underwent maintenance or something. GIMPS has probably gotten the ECM up to [B]t60[/B] or
beyond, so there's little hope of factoring it anytime in the near future.[/QUOTE]

Thanks. What does t60 mean, to ECM?

Thought I had lucked onto a big special form prime. Not this time.

[QUOTE=davar55;306978]Thanks. What does t60 mean, to ECM?

Thought I had lucked onto a big special form prime. Not this time.[/QUOTE]

txx is a commonly used term to mean enough ECM has been run up that it's 1-[URL="http://www.wolframalpha.com/input/?i=exp(-1)&t=crmtb01"]exp(-1)=~%67[/URL] likely that there's no factor less than xx digits. Less commonly seen is something like 2t60, meaning there's a [URL="http://www.wolframalpha.com/input/?i=exp%28-2%29"]exp(-2)[/URL]~=13.5% chance of having missed a 60 digit factor.

From [URL="http://www.mersenneforum.org/forumdisplay.php?f=96"]YAFU[/URL]'s docfile:
[quote]------------
T-levels
------------

A note on the "t-level" terminology used in factor(). Something that has
received, say, "t30", has had enough ecm curves run on it so that the probability
that a factor of size 30 has been missed is exp(-1) (about 37%). Likewise,
t35 indicates that factors of size 35 are expected to be missed about 37%
of the time (at which point a 30 digit factor would only be expected to be
missed ~5% of the time). t-levels are calculated from tabulated data extracted
by A. Schindel from GMP-ECM in verbose mode. See also the GMP-ECM README file.[/quote]

Looking at PrimeNet's [URL="http://mersenne.org/report_ECM/"]ECM Progress[/URL], M1901 has been taken to t50, and has around 17% of the necessary work to get to t55.

1 - exp(-1) =~ 0.63 = 63%