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Diary, July 2009

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Wednesday, July 1, 2009

Playing Go

This evening, I went to the University by bike to play Go. When I arrived Rudi, Huub, Araldo and Taco where already sitting outside on the sideway café. I joined them and soon Araldo invited Taco for a game, and Rudi announced that he would be watching us play. Huub and I agreed on taking three stones ahead. I thought it was too much and said that it would be his blame if I would lose, which made the other laugh. Huub and I played quite fast, and at one point I could have killed one of the groups of Huub, but I failed to realize how, and played a Ko instead. In the end I lost with 48 points. Then we talked for some time and Rudi and Huub took some pictures and we went to listen to some people from the Drienerloos Vocaal Ensemble who where singing from Weihnachtsgeschichte by Hugo Distler in the exhibition room which has quite a strong (but not too strong) resonance. Then we watched the game of Taco and Araldo. I left before the game was ended, but did make some suggestion to Araldo just before I left, while we were discussing some possible moves.

Thursday, July 2, 2009

Water wells

Yesterday, I received some message about an inquiry I had made to someone of the Dutch Geology Society in Twente about the natural water wells along the road to my office. It seems that the first of the wells, had disappear after some groundwork was done. It is possible that a drainage pipe was installed. The other two wells have dried up in the past months, which happens more often. The messages explains that during the construction of the Twentekanaal around 1930 the area where the road lies has been excavated for about five meters (15 feet) and the slope that was thus created, probably cuts through some water bearing layers. I fully agree with this explaination because there is actually quite a lot of reed growing on various parts of the slope. Which in a way is remarkable in itself, because usually you do not find it in such a spot.

Friday, July 3, 2009

Drop in temperature

This afternoon, between four and five o'clock the temperature dropped with 10 degrees Celsius. See this graph of the temperature during the day and the temperature profile at four o'clock of the Netherlands as a proof. This was all caused by a row of thunderstorms crossing the country. When I biked home, I saw some flooded streets.

Sunday, July 5, 2009


This afternoon, I discovered a single flower in our Magnolia. I took a picture of it.

Wednesday, July 15, 2009

Playing Go

This evening, I biked to the appartment of Taco to play Go. I was the first to arrive. After some time, Rudi arrived, and just when we decided that he would play simultaneously against Taco and me, Araldo arrived, and it was decided that I would play against him. We agreed on a six stone handicap for him. Just a few moves into the game, Huub also arrived, and he watched us play. He did take some pictures. In the end, I lost with four points. (Game record of the first moves.)

Sunday, July 19, 2009

Exact Cover

This weekend, I worked on a possible improvement on my Exact Cover program. I tried to generalize the reduce algorithm, which sees if there is an implication relations between two columns. If this is the case, the vectors for which only the implied column is filed can never be selected, and thus be eliminated. The idea is to take a subset of columns and solve the Exact Cover for this subset. Vectors that are never used, can be eliminated. The problem is that the number of possible subsets is very large, to be exact two to the power of the number of columns. And the larger the subset, the more time it takes to calculate the Exact Cover. It took me some time to implement a selection algorithm and to program the rest of the algorithm. I tested on several Exact Cover problems, but the results where rather disappointing, because not many vectors where eliminated. For this reasons, I decided to put the new code under an additional option (-reducegroup). The program can be found here.

Monday, July 27, 2009

Dune mini-series

Last weekend I watched the mini-series Dune and Children of Dune on DVD. It is clearly that the available budget of the second series was much larger than the first. The special effects of the second series are much better than the first. But for the rest, I find both adaptations quite good and enjoyable to watch. In the second series there are some scenes where Reverend Mother Mohiam lays some hexagonal tarot tiles. I did search the internet to see if I could find images of the full set, but the only thing I found, was the image shown here, which seems to be a still taken from the DVD.

Tuesday, July 28, 2009

Exact Cover and ZDD

Last Saturday, I paged through Volume 4 Fascicle 0, Introduction to Combinatorial Algorithms and Boolean Functions by Donald Knuth (ISBN 0-321-53496-4) (of which an earlier version can be downloaded from The Art of Computer Programming page), when I saw some mentioning of Exact Cover and ZDDs (Zero-suppressed Binary Decision Diagrams). This brought me about studying ZDDs. After some studying, the material is not always easy to understand, I realized that constructing a ZDD of an Exact Cover is nothing else than creating a DAG (Directed Acyclic Graph) representing all possible solutions. Although the construction of a ZDD is rather straightforward, it turns out that the size of the ZDD strongly depends on the order in which the vectors are selected. So far, I haven't found anything about a generic method for selecting the vectors. I have concluded that this approach is only feasable for situaties when an Exact Cover has large number of solutions, such as for example calculating the number of domino tiling on a 8 by 8 square. But there are also other methods for calculating these, for example by calculating the recurrence equation for the number of Domino Tilings in P8 x Pn as given here.

This months interesting links


KMZ file of movements.

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