Wednesday, 18 December 2013

December of '13

Its almost the end of the year and i dont know what i have really gained in these holidays or perhaps, lost. I am currently attending this workshop on Classical groups and it has been a nightmare. I enter the class on the very first day of the conference and am really un-intimidated by the style of people, anupam reading Sury'
s limerick which was kinda funny,the way Anupam said it, director giving a nice intro to the conf. folks and the most funny part in his speech was something like the following. :" i know you are mathematics people,theoretical people, but come visit our labs". and i could not stop grinning like a kid who knows what the other naughty kid is upto. well then the conference started with introductory lecture of Shripad of which i did not make out anything, later anupam started with basic group theory concepts and whatever we had learnt in an entire course in group theory,anupam summarised in 5 minutes and then the fun part started. We know the definition of a group action and obiously as though emphasised by Chandrasheel, it is more important that it looks like,group action is used like all the time. coming back to the topic,we know what a transitive action is, action which has only one orbit. now comes the stuff i learnt or so i think. we define something called a doubly transitive action in which a pair of set elements go to some other pair of set elements by the action of some group element. this is a stronger version of transitivity and hell it is more important than it seems. now comes the concept of a block(B) in the set on which the group is acting, it is a proper subset which satisifies a property that any group element acting on the subset B leaves it invariant setwise or creates a new subset which is disjoint from B. Now if a group acts on a set such that there are no blocks then the action is called primitive. And primitivity is an important notion. The only connection between doubly transitive action and primitivity is that if the action is doubly transitive then it is primitive, this can be proved easily. Later comes the core theorem called Iwasawa theorem which is basically a machinary to prove that some group is simple. Why should we care whether a group is simple or not. we will come to that question later. What exactly does Iwasawa theorem state. it states that if the group action is transitive and primitive and that there is a normal subgroup in the stabilizer of some set element such that the ......then the group is simple. pardon me. well later in the book which we are following or rather stopped following is Larry Grove's 'Classical groups and Geometric algebra'. the idea is simply to find simple groups. and we find new simple groups which i had never seen before.groups which are created by going into projective geometry and then groups which are isometries over an alternate bilinear form or a symmetric one,with some modification to the group.  i stopped understanding since the beggining of this week.we had some fantastic mathematicians coming and giving a talk like George Lutszig and MS ragunathan today who had met and had several correspondence with the great mathematician Andre Weil(who literally started calling some groups as classical groups and the terminology stuck) and Grothendieck himself. Some important person are yet to come. I think i will stop visiting the workshop coz i dont understand anything these days,today in raghunathan's lecture i was studying the ring theory from artin. and i dont seem to making any progress,as always. hermann weyl was a colorful character as raghunathan was recouting from his meeting with weyl in tifr in 60s. i think i want to be a mathematician. the only subject where i see theoreitcal buildup,whatever that means, is in mathematics. they define objects abstractly,they deduce somethings,they make more structure, they deduce some more, just to fit the picture,they bend the structures,they twist 'em and later remove the entire pile. several such instances and you get a good definition and then you work on it. i have not seen this in any other subject and i love it. well sometimes i hate it too.
well apart from the conference, i wasted my entire december. i should have learnt or at least revised linear algebra,done some group theory problems, done some analysis.lately i have started running from my hostel to puc and back. i have been consistent about it in december except maybe for 3 days. i hope to continue it but considering my lack of willpower i really dont know.
i completed Boardwalk empire,all 4 seasons and also Breaking bad,all 5 seasons. liked both of them.
this december i was supposed to attend this movie making workshop being held here but i joined the classical groups camp and i can't do movie making and yet i do not regret but i would have been little less sad if i could do both. 

Sunday, 8 December 2013

Little bit of everything

We as Indians are a peculiar figure not only to the people living in other countries whose perceptions might be prejudiced by the notion of the Orientals and brown charming people who live in shitty conditions and are overflowing in almost every other goddamn country but we are peculiar to ourselves as well. When the daughter of the neighbour is talking with the unknown guy in low-waist jeans, i will feel the urge to share it with everyone and let them know that all the vices in the world originate from this very phenomenon but when my daughter is doing the same, i will call her quietly and try to get sense in her otherwise i will publicly disown her. my job as a parent in limited only till the offspring takes heed of me otherwise we will kill her. Coming back to the relevance of topic,when we Indians, are at a holy ceremony or public weedings(this is deliberate) or some kind or any kind of buffet where we get to choose from a variety of dishes.Point to note is that these kind of occasions did not arise in the past when people used to sit on the floor and some people used to serve so that the dishes were limited and also the money to arrange for some random people to serve the dishes. Then came the idea from west ,why do people need to be given the food, they can take it themselves from some facility which would allow them to not waste food as well. We as indians did not realise the idea of this concept. As i started moving among the crowded line of people flocking the side of a long buffet table, i found a plate and i took it and i found a spoon and i took it. I found a bowl and i took it. I started moving to the real section. i saw salad and took a chunk of it so that if i had to survive on it, i could but also judiciously disposing it to a small section of the plate, i found the pickle and i took a chunk of it,what if there were no curry and i had to just suffice with pickle. i found the chapati and i took 4. normally i would eat 2 but many a time when i get hungry i can manage 4. as the line kept moving gradually i was making sure i miss nothing. i got curry which was literally overflowing my bowl. i moved on . i saw paneer. viola. hell i took only i bowl. i do not move back and get a bowl, i ask the guy beside me to ask the guy besides him to ask the guy beside him....to the guy currently near the bowl section to pass a bowl. by the way,this is the usual way we transport anything. this is called Chained serial  automatization. i move on with liberal amount of paneer in my bowl. then i saw dal, i did not have bowl but i moved on being in high profile party and i did not want people to think that i was coming from food-stricken places.my plate was already full. then i saw bhindi i resisted my temptation while people in front and back took liberal amounts of it.i saw kaju curry,i dont like it these days coz who the hell puts kaju in a sour dish.i saw aloo gobi, anyways not my concern,i dont like it. i saw desert, gulab jamun ,there was no place on my plate,i moved on , i saw custard. who the hell keeps two types of desert in a single occasion. i had to move on. then finally the buffet table ended. i looked for a place to sit on. i found a lonely spot and moved on to it. my mind was fighting terrible battles over my foolishness. i was such a big fool. anyways, its just a party there will be many more,i should probably finish my food and go back home.
i went out to the washroom, flushed my hands, combed my hair.
now i was practically a new person, i left the loo, joined the buffet line,this time,taking a little bit of everything.

Tuesday, 22 October 2013

The probability of innocence



An interesting application of simple probability theory came when the Florida police department had to prosecute a defendant for buying an illegal drug, cocaine. In the US, both buying and selling cocaine is a criminal offense.
One night Florida police raided a suspicious facility, upon finding nobody, they just seized the bags lying around. Out of the 496 bags confiscated, they randomly chose four and found them to be cocaine which were immediately destroyed (not the whole batch, just the four tested to be truly cocaine). Come after few months, they randomly chose 2 packets from the lot and sold it to a person posing as drug dealers. During the same night, they arrested the person for crime of buying cocaine. They could not find the sold packets in the person’s house and neither in his body (he tested negative for drug intake).In the court the matter arose ‘What if the other packets were not cocaine’, how could the person be prosecuted without any evidence. Florida Police consulted the local statistician and asked the probability of the person being innocent. Turns out the answer could be computed without much difficulty.
Out of the 496 packets, say N are packets of cocaine and 496-N are something else, say X (which is not illegal). The probability of the person (henceforth defendant) being innocent is the probability of the event in which the defendant bought X which would be the case corresponding to the situation that Florida police chose 2 packets of X from 496-4=492 bags and sold it to the defendant.



Since N could be any number from 1..496 we will try to estimate N. The court being an authority of fairness would not convict the defendant without any strong reasons. Shuster computed the maximum likelihood estimate of the given event i.e. what must be the number N such that it maximizes the probability of innocence, which is fairly easy to compute.
Maximizing P is same as maximizing log (P) (since log is a 1-1 function). 


This function can be numerically optimized w.r.t N and since those days (which was in 1991) had good computers, the result came out to be N=331 and the probability of such an event occurring (by plugging the values back) is 0.022 which is a modestly lesser than the probability of an event in which there are 100 men (including one criminal) and you point out a random guy and proclaim him to be the culprit. Hence the defendant was prosecuted by modest application of probability theory in real life.

 References:
1.       Shuster, J.J (1991).The Statistician in a Reverse Cocaine Sting. Amer. Statist, 42,203-204.
2.       Statistical inference, Casella and Berger, 2nd edition, Duxbury Advanced Series.