28 Mar 2011 dangermaus   » (Journeyer)

Some topics in maths I did not knew or refreshed with Khan Academy are: Laplace Transforms, Convolution Theorem, Dirac Delta function, Taylor Theorem, Implicit differentation, insights in Chain rule, Conic Sections (foci of Ellipse, Hyperbola, and Parabola with Directrix), partial fraction expansion, l'Hôpital rule, surface integrals... I also learned a bit finance on forward contracts, futures and the margin mechanism I can use on the job. Sal, you are great!

Useful git hints

These git hints are taken from the Linux Magazine, article "Git in der Praxis".

  • Initial configuration:
    git config --global user.name Danger Mouse
    git config --global user.email dangermaus@users.sf.net
  • Coloured output:
    git config --global color.diff auto
    git config --global color.status auto
    git config --global color.branch auto
  • New repository:
    cd myproject
    git init
    git add .
    git commit
  • Ignoring files: put your entries in a file .gitignore
  • Clone Linus kernel's repository:
    git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6.git linux-2.6
  • Push changes to repository:
    git commit -a -m "these are the changes"
    git push
  • Retrieve changes from repository with git pull
  • List existing branches: git branch
  • List existing tags: git tag
  • Create new branch:
    git branch Refactor
    #switch to branch
    git checkout Refactor
    #back to master
    git checkout master
  • Create new branch and switch into branch:
    git checkout -b Refactor
  • Merge from branch Refactor into master:
    git checkout master
    git merge Refactor
  • Create a tag:
    git tag -a "Tag_1.3.6" -m "new tag at version 1.3.6"
  • Retrieve uncommitted changes with git status
  • List latest commits with git log
  • Create a patch:
    # on branch Refactor
    git commit
    git checkout master
    git pull
    git checkout Refactor
    git rebase master
    # repair conflicts
    git commit
    git format-patch master --stdout > mybugfix-patch.diff

I am also trying to understand a bit of this book:

Indra's pearls

The book is centered around Möbius transformations and Schottky circles. Möbius transformations

f(z) := (az+b)/(cz+d)
are visualized as compositons of affine projections (az+b) and of projections on the Riemann's sphere 1/(cz+d) of the complex plane. They are at the ground of Beltrami disk (on page 379) and represent a non-euclidean geometry for which the parallel postulate is not valid. Schottky circles are generated by two Möbius maps which project one circle a into a circle A and another circle b into a circle B. With a breadth-first or depth first algorithm all set of reduced words (e.g AbBb reduces to Ab) are generated, and interesting pictures come out. Like the glowing gasket by Apollonius, invented in 220 BC, which is also the best way to pack as many tangent circles as possible (apollonian packing). The most famous fractal (Mandelbrot's apple) shortly appears at page 291.

Debian Squeeze

When installing Debian Squeeze, the partition with Windows XP was not recognized. Normally I adjust /boot/grub/menu.lst but this time the file was missing.

did it....

As "scientist of information", you might find these tricks with M$ tools useful:

Creating SQL statements out of an Excel table with data
An insert:
=CONCATENATE("INSERT INTO tbdata(id, name, address) VALUES(";A1;"'";A2;"'";"'";A3;"'";");");
Or an update:
=CONCATENATE("UPDATE tbdata SET name='";A1;"');");

Lookup table in Excel

This formula looks up for the value in C3 in the table $L$2:$O$25 and returns values in the 3 column where C3 was found. It's a vertical lookup.

Importing Excel tables into access With File->Import->LInk External Table it is possible to import Excel tables into Access and then with SQL queries to extract data from them. However, I learnt that there is a limitation in the complexity of queries you can launch. I always switch from 'Design view' into 'SQL view' when creating queries.

For the interoperability of the Deltasql project, I setup a MS SQL server 2008 instance...

Installing SQL server

Downloading and installing it is easy, but at the end you need to assign a port to it, else the thing won't work. Might sound simple, but I assumed this step was automated by the install procedure. At best, you install it without touching defaults and as last step tweak it to open say port 2345 on TCP/IP protocol. Only after this step, you'll be able to connect with SQL Server Management studio client.

I still think that Oracle and Postgres are the high end of the database offer. However, I see once again how Microsoft is eating more and more of the lower segment. I know many little companies (10-30 employees) which now use some accounting solution based on SQL server. Still, sometimes at work I am puzzled and wonder if the transaction integrity of SQL server is always guaranteed, when under heavy load...

Tips to install Gentoo

I used an old CD to reinstall the latest Gentoo on andromeda, but failed due to some kernel issue with udev after several tries. I then decide to install Debian Squeeze, as I did not want to recompile everything on this old toaster (1GHz, 512 RAM) :-) As side effect I learned that it is always best to use the latest image of Gentoo install CD to get it running 8-)

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