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03.04.2006 Monday - windy stop (with a smile)

:)



02.04.2006 Sunday - diary and some goodies

Yesterday we have been in Florence and returned really late. Couldn't sleep this morning, so woken up early. In the first minutes thought that because of the missing hours of sleep and because of recent developments it would be a bad day. ...but after opening the window discovered that there was a fantastic Sun outside: been waiting for this since several months.

Never undestimate the energy emitted by the Sun: a ball of plasma with a diameter of 1.392 million km and a mass of about 2.0x10^30 kg. Approx 5500 K at surface... ... man ... that's HOT :D

Etherea sent me an english translation of my CV. Thnx Ethy :******

In her blog I have also found this funny image of Iakko :D

(Hum.. Cle... there is no way to link your blog directly without loosing the external frame)

While getting my coffee I've added a couple of comparison operators to the SSMatrix class. The scalar ops are still missing, but the sun is calling me thus I'll add them the next time :). I've also upped the first SSComplex template that can be used with SSMatrix. Enjoy :)

It seems that there is some new movement around KVirc.

For chronicle purposes I also to need to record the following (important) note.

Segmentation fault. Core dumped.

"Hum... still need some debugging"



01.04.2006 Saturday - April Fools

1st April. In many countries this is the day of "Jokes". "Prima Aprilis: uwazaj bo sie pomylisz".. which is in Polish and means "1 April, be careful because you might be wrong". "April Fools" and "Pesce d'Aprile". The (net-)jokes already started.

Heh.. after you read some of them, all the of the web starts to look like a joke :D

An Insightful post on slashdot stated:

Try date -u and you will see that it is in fact April Fool's Day.

Note that April Fool's Day, as defined by the International April Pranksters Association, goes by UTC, not by local time zones. Because IAPA is not widely recognized as an international standards body and many people's problems to understand time zones, this has led to some problems since its introduction in 2002. It is especially uncertain for regional publications, which are reluctant to adopt the new standard because they fear to irritate their audience. One example is the Hubsborough Gazette, which famously spread confusion on the evening of March 31st, 2004 (EST) when an article claiming that aliens have attacked the Whitehouse appeared on their website. Despite the seemingly obvious nature of the hoax, many believed it and called the authorities or local clergery for guidance. One family even is reported to have spent two weeks in their backyard bunker. Since then, the editor has announced that they will only publish April Fool's articles during the hours when April 1st of their local time zone and UTC overlap, and take down articles afterwards. Many publications have followed their example in the following years.

The guys from Centrica fooled me about bex2 emitting all zero size output... that would be a problem. It is a 1st April joke..isn't it ? :D



31.03.2006 Friday - diary and other goodies

Hum.. a lot of images (in random order):

  • The Diophantine Equation
  • A nearly black cat
  • The Youla-Kucera theorem (kinda hard proof for the multivariable case)
  • A pair of sweet lips :)
  • The missed Frantz sister's party: mea culpa :(
  • Francesca in the lecture room
  • Silvia pressing for messaging
  • Samuel spending up to the last cent for a drum set
  • The two girls near Skile tripping three times around the block
  • A sensation close to deja-vu
  • Iakko laughing hard after drinking 4 Hoegarden glasses :D
  • ...
Nice four days :)

The promised LU decomposition code is here. Pretty straightforward once the formula is known. The file contains also the algorithms for matrix determinant (computed either by LU or by a recursive adaptation of the Liebnitz fomula) and for matrix inversion. Enjoy :)

"I have a monster in me and if I wake him up he is probably going to kill me"



27.03.2006 Monday - The LU decomposition of a matrix

Back to my fast determinant computed by using the LU decomposition of a matrix.

The idea is to decompose the matrix A in a product of two matrixes L and U in that L is lower triangular and U is upper triangular. We then know that det(A) = det(L) * det(U) and the determinants of triangular matrixes are trivial to compute. The LU decomposition is not unique so we can finally choose some arbitrary elements of L or U. We choose the diagonal of L to be made of all ones so the determinant of L is directly computed as 1.

Determining L and U such that A = LU is a system of m^2 equations (where m is the rank of A) in 2 * m^2 unknown variables. The bounds of L being lower triangular with a diagonal of all ones and U being upper triangular reduce the number of unknown variables to m^2. The system looks like the following one (3 x 3 case):

Which can be expanded to:

This shows us that we can compute each row of L and U independently by solving a m-equantion in m-variables system. In the 3x3 case the three systems look like:

Which can be solved by inverting the coefficient matrix or by Gauss-Jordan manipulation. The Gauss-Jordan solution for the 3x3 case looks like:

There's obviously a pattern in the solution vector and it can be written down as follows:

Now we have a LU decomposition and can compute the determinant just by multiplying together the diagonal elements of U. Nice eh ?

This is pretty fast. While the Liebnitz method costs like n!, the LU method costs like n^3 which is faaaaaar bettter :)

Now one should prove that the LU decomposition exists for every non-singular matrix and verify the numeric stability of the method... I'll do it in the next days while implementing this stuff and report you later.



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