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Author: lievenlb

nothing beats the command line

Published March 10, 2004 by lievenlb

Over
the last couple of days I’ve been experimenting a bit with different
backup methods. To begin, I did try out ExecutiveSync and its
successor You Syncronize but they are very, very
slow. Not only did the first synchronizing of a 0.5 Gb Folder between
two computers over our Airport-network took over 2.5 hrs, but also on
subsequent syncs the checking of the database seems to last forever.

So I turned to the fink project
again and did find two interesting packages : wget . GNU Wget is a free network utility to
retrieve files from the World Wide Web using HTTP and FTP, so one way
to backup a folder would be to put it in the Sites folder and
mirror it over the network using wget. I did’t check this out in
great details (did a small test to see it working but I assume it will
be slow for large folders). The other one is rsync It uses the “rsync algorithm” which
provides a very fast method for remote files into sync. It does this by
sending just the differences in the files across the link, without
requiring that both sets of files are present at one of the ends of the
link beforehand. This seems to be precisely what I wanted to do and
after a google for ‘rsync OS X’ I arrived at the RsyncX package which is an implementation of rsync
with HFS support and configuration through a command line (Terminal) or
graphical user interface. I downloaded this package and the GUI seems to
be placed in the Applications/Utilities and tried it out by
filling out the Source and Local Folders and pressing the synchronize
button. Not much progress was reported but the Activity Monitor
showed that it was using up all of the CPU so I was patient for over an
hour and then looked for the Network Activity in the Activity
Monitor and virtually no packets were going in or out, so I killed
RsyncX. I am sure I did something wrong but rather than trying to
get it working, I tried the command-line rsync-command I
downloaded from Fink. After a few false attempts I
typed

/sw/bin/rsync -a -e ssh
iMatrixLieven.local:/Users/lieven/Documents
/Users/lieven/docsLieven

and suddenly the packets were flying
happily over the network at 250 Kb/sec, so it took me only half an hour
to get a first synchronization done and subsequent changes are added in
no time! Afterwards I discovered that rsync is included in the
standard OS X Developers Tools as RsyncX seems to have replaced
it to rsync_orig and installed a new (quite large) rsync
in /usr/bin. Maybe my problems with RsyncX were caused
because I have /sw/bin earlier in my $PATH than
/usr/bin but verifying this will have to await another day. For
the moment, I’m happy to have a quick syncronizing tool available and
Real Madrid is playing on the TV…

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noncommutative geometry 2

Published March 9, 2004 by lievenlb

Again I
spend the whole morning preparing my talks for tomorrow in the master
class. Here is an outline of what I will cover :
– examples of
noncommutative points and curves. Grothendieck’s characterization of
commutative regular algebras by the lifting property and a proof that
this lifting property in the category alg of all l-algebras is
equivalent to being a noncommutative curve (using the construction of a
generic square-zero extension).
– definition of the affine
scheme rep(n,A) of all n-dimensional representations (as always,
l is still arbitrary) and a proof that these schemes are smooth
using the universal property of k(rep(n,A)) (via generic
matrices).
– whereas rep(n,A) is smooth it is in general
a disjoint union of its irreducible components and one can use the
sum-map to define a semigroup structure on these components when
l is algebraically closed. I’ll give some examples of this
semigroup and outline how the construction can be extended over
arbitrary basefields (via a cocommutative coalgebra).
–
definition of the Euler-form on rep A, all finite dimensional
representations. Outline of the main steps involved in showing that the
Euler-form defines a bilinear form on the connected component semigroup
when l is algebraically closed (using Jordan-Holder sequences and
upper-semicontinuity results).

After tomorrow’s
lectures I hope you are prepared for the mini-course by Markus Reineke on non-commutative Hilbert schemes
next week.

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tweedledee and tweedledum

Published March 8, 2004 by lievenlb


Tweedledum is a first-generation iMac (233 MHz
slot-loading, 192Mb RAM, No Airport) whereas Tweedledee is
2nd-generation (350 MHz front-loading, 192Mb RAM, Airport card). A
couple of weeks ago I replaced their original hard-discs (4 Gb resp. 6
Gb) by fat 120 Gb discs and from this weekend they serve as our
backup-facility. Tweedledee is connected via Airport to our network and
is a fully functional 10.3 computer, everyone has a login on it and is
encouraged to dump important files onto it as a secondary copy.
Tweedledum. on the other hand, is invisible to the network but forms a
one-wire network with Tweedledee (they are connected by a crossed
ethernet cable which results in having a self-assigned IP address in the
169.254 range and hence they can see each other; moreover using the
Sharing-pane in the System Preferences I allowed
Tweedledee to share its internet connection to other computers,
connected to it via Ethernet, so Tweedledum can go online to get
system-updates when necessary).
A house-computer rule is that
all important files (which means those you don’t like to loose in a
crash) are kept in the Documents folder of your
Home-folder on your own computer. At regular intervals I make
sure that these folders are synchronized with backup-copies on both
Tweedledee & Tweedledum, so at any given time there are at least 3
computers containing the essential files (usually more as everyone has a
login at each of the 4 ‘work’-computers and can drop extra copies
around, but must clean-up when asked).
To synchronise I use
the shareware program ExecutiveSync. It is no longer possible to
obtain this from its original homepage as they seem to have been taken
over and invite you to buy You Sinc instead which costs more than
twice what ExecutiveSync costs (19.95$). Fortunately, for now you can
still download it from the Apple site. I have
ExecutiveSync running on Tweedledee (you are only allowed to run it on
one computer, you can install it on every computer but then the
synchronizing process is sometimes not possible which is why I came to
the following work-around). In ExecutiveSync you make several
Projects which involve choosing a Local folder and a
Remote folder somewhere on your network which you want to keep in
Sync. In my Home folder on Tweedledee I made several (originally
empty) folders such as docsGitte. Then my ExecutiveSync-project
syncGitte takes docsGitte as the local folder and the
/Users/gitte/Documents-folder on iBookGitte as the remote
folder. The first time you synchronise takes a lot of time (especially
over the wireless network, it may be better to do the first sync via
ethernet) but afterwards it works pleasantly.
Once I
synchronised all the local Documents-folders with the corresponding
folders in my home-folder on Tweedledee, I have another
ExecutiveSync-project BACKUP which takes as the Local-folder my
Home-folder and as the remote folder a folder BACKUP I did create
on Tweedledum. Fortunately, here the synchronising is done over Ethernet
or it would take forever.

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