throwaway13qf85 · HackerTrans

throwaway13qf85

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A Futile War on Drugs that Wastes Money and Wrecks Lives

ft.com

1 pointsby throwaway13qf8512 anni fa0 comments

Why are there exactly 12 pentagons and 20 hexagons on a soccer ball?

math.stackexchange.com

3 pointsby throwaway13qf8512 anni fa0 comments

comments

throwaway13qf85

12 anni fadiscuss

Intuitively, it is to indicate degree of 'truthiness'. Certainly false {F} is less truthy than either both true and false {T,F} and neither true nor false {}, which are in turn less truthy than certainly true {T}. The Hasse diagram indicates that there is no relationship between {} and {T,F} in terms of level of truthiness.

More mathematically, you can think of the Hasse diagram as a lattice, with the meet and join operations being AND and OR. For example, we are used to thinking of the expression

  p AND q

as being true if p is true and q is true. But what if we have four values: {}, {T}, {F} and {T,F}? The Hasse diagram tells us how to interpret expression like this - p AND q is the meet (greatest lower bound) of p and q, and p OR q is the join (least upper bound) of p and q. So for example,

  {F} AND {T}   = {F}
  {F} OR  {T}   = {T}

  {F} AND {}    = {F}
  {F} AND {T,F} = {F}
  {F} OR  {}    = { }
  {F} OR  {T,F} = {T,F}

  {T} OR  {T,F} = {T}
  {T} OR  {}    = {T}

  { } OR  {T,F} = {T} // I think..!
  { } AND {T,F} = {F}
  { } OR  { }   = { }
  { } AND { }   = { }

The Hasse diagram in the article makes sure that expressions like this agree with our intuition in the cases where we have intuition for what the result should be, and give us a way to interpret expressions consistently when our intuitions fail us.

We're used to seeing Hasse diagrams for subsets of a set S used to indicate inclusion, in which case you would have {} < {F}, but you can have a valid poset structure that's not based on inclusion.

throwaway13qf85

12 anni fadiscuss

In case anyone is turned off by the expectation of wishy-washy flim-flam, it's worth noting that Graham Priest may be a professor of philosophy, but is most well known for his work in logic, particularly in non-standard logics (i.e. not propositional or predicate logic) that allow non-boolean truth values, or weaken or remove some of the axioms of classical logic.

I assume that what's relevant here will be his work on paraconsistent logics (which allow contradictions) but I think an equally interesting line of work is linear logics. It's of interest to computer scientists because of its relationship to linear type theory (in the same way that the Curry-Howard correspondence links type theory and classical logic) and because it has close relationships with quantum computing.

In particular, you are not allowed to delete or duplicate elements/propositions/types (corresponding to physical processes that cannot arbitrarily create or destroy particles, the "no deletion theorem" and "no cloning theorem" of quantum information theory), so functions of the type

  duplicate :: a -> (a, a)

and

  delete :: a -> ()

are not allowed in a linear type system. Practically, this means that many operations can be optimized by the compiler to in-place mutations, because it is guaranteed that there is only ever one reference to a particular object.

throwaway13qf85

12 anni fadiscuss

> Do people ever go to poor areas to enjoy themselves?

Absolutely! There are many poor areas in London (Whitechapel, Brixton or Dalston, for example) but the idea that you would have to avoid them is crazy. In fact there are great reasons to visit them - the best Indian or Pakistani food in the city is in Whitechapel, the music scene in Brixton is amazing, there are great clubs in Dalston if you don't mind the hipsters.

How has SF let things get so bad that there is a major area of the city that people are told to actively avoid because of all the homeless people?

throwaway13qf85

12 anni fadiscuss

So your advice is - to enjoy SF, stay in the expensive areas, and get a taxi if you need to go through the poor areas, or the parts where there might be homeless people?

throwaway13qf85

12 anni fadiscuss

I don't think you understand what the phrase "elephant in the room" means. If lots of people are talking about something (and certainly, a lot of people are talking about the tech sector vs the rest of the economy) then it's not the elephant in the room.

throwaway13qf85

12 anni fadiscuss

Nice work. There's a similar article which uses Haskell as the base language instead of Python, which is also worth reading (this is actually how I learned both Haskell and Scheme..!)

https://en.wikibooks.org/wiki/Write_Yourself_a_Scheme_in_48_...

throwaway13qf85

12 anni fadiscuss

William Faulkner once criticised Ernest Hemingway by saying that he "had never been known to use a word that might send the reader to the dictionary."

Hemingway responded by saying, "Poor Faulkner. Does he really think big emotions come from big words? He thinks I don't know the ten-dollar words. I know them all right. But there are older and simpler and better words, and those are the ones I use."

throwaway13qf85

12 anni fadiscuss

To explain the flaw in the reasoning, it's easiest to generalize -

    y = x ^ (x ^ x ^ x ^ ...)
      = x ^ y

and hence

    x = y ^ (1/y)
      = exp( 1/y * log(y) )

which is defined for y > 0.

Graphing this using Google plot [0] or Wolfram Alpha [1], or just differentiating, reveals that it has a maximum at x = e, and that x takes the value sqrt(2) at both y = 2 and y = 4.

Therefore the inversion, which is the original equation

    y = x ^ (x ^ x ^ x ^ ...)

is dual-valued, i.e. it is not a mathematical function. For it to be a function, you need to specify whether you are on the upper branch (so that y = 4) or the lower branch (so that y = 2). Deliberately obfuscating the difference between the two branches leads to the conclusion that 2 = 4.

The square root function can lead to a similar confusion, since there are two solutions to the equation

    y = x^2

and hence the "function"

    x = sqrt(y)

is not really a function unless we specify whether we are on the upper (x > 0) or lower (x < 0) branch. By convention we interpret sqrt(y) to be the upper branch, and write -sqrt(y) for the lower branch, but there's nothing that forces that choice.

Obfuscating the difference between the two branches could lead one to conclude that 1 = -1, although the fallacy is more obvious in this case, since everyone is familiar with the fact that the square root function is dual-valued.

[0] https://www.google.co.uk/search?q=y%5E(1%2Fy)

[1] http://www.wolframalpha.com/input/?i=y%5E(1%2Fy)

throwaway13qf85

12 anni fadiscuss

This is very much a beginner's book. When I interviewed for internships in the summer of 2008, the interviewers expected that I would already be familiar with a large portion of the content of this book.

throwaway13qf85

12 anni fadiscuss

I don't think there's anything wrong with what they say. Your chance of winning one round of RPS is 1/3. Out of three games you win once, lose once and draw once on average.

If you replay when a draw occurs, then of course you have a 1/2 chance of winning.

throwaway13qf85

12 anni fadiscuss

To clarify for the poster who asked you the question, playing aggressively might mean better more than your hand otherwise dictates, but it might also mean betting more often even if you don't change your bet size.

For example, you might always fold low pairs 22-66 initially (and perhaps conspicuously reveal that you folded them) but then start raising with them later. If your opponents were paying attention, then (a) they will think you have a good hand, and may be scared into folding right now, but (b) even if they don't, they won't suspect you of having a low pair, so that if e.g. a 2-6 comes up on the board and gives you three of a kind, you have an advantage because your opponents won't consider that you have this hand.

throwaway13qf85

12 anni fadiscuss

Is that the Georgia Tech/Tucker Balch course?

I'm amazed that GT let him teach that course. It's full of errors, omissions and bad habits. I vaguely followed along, and the most noticeable thing was how much the students complained about the low quality of the course.

The only result of someone implementing the advice in that course is that they'd lose a lot of money quite quickly.

throwaway13qf85

12 anni fadiscuss

It really depends on what you're trying to achieve. The skills and knowledge necessary for long-term equity investment, short-term statistical arbitrage, derivative pricing and high-frequency trading are all quite different.

throwaway13qf85

12 anni fadiscuss

The page you linked to is Dimensional explaining their five factor model, and it sounds like they only use that as a basis for a more complicated model.

The very example you gave to support your point actually detracts from it.

throwaway13qf85

12 anni fadiscuss

I don't think I would call CAPM, the Fama-French 3-factor model, VaR, Black-Scholes and GARCH "the best of the best".

I would probably call the selection of topics "finance basics that no one in industry uses any more".

It may be suitable as a beginner's guide if the quality of the exposition is good enough, but my suspicion is that you would be better off getting a copy of "Python for Data Analysis" and a decent quant finance textbook.

throwaway13qf85

12 anni fadiscuss

I would be more forthright - the Haskell book was a travesty that never should have been published and I wouldn't want to inflict it on anyone (I have experience with all three of Haskell, quant finance and modelling & analytics and I am confident in saying that the Haskell book is useless).

throwaway13qf85

12 anni fadiscuss

Does anyone know if this is any good? I don't think that Packt is a particularly reputable publisher. They published a similar book about Haskell (called "Haskell Financial Data Modelling and Predictive Analytics") which was absolutely terrible - useless for learning anything about Haskell, finance, data modelling or predictive analytics.

You can find a commentary on that book here on Reddit[0].

I hope this book is better, but I will withhold judgement until someone comes along who's read it and is able to give an informed opinion.

[0] http://www.reddit.com/r/haskell/comments/1rj2jq/book_haskell...

throwaway13qf85

12 anni fadiscuss

I'm not going to claim that it improves my creativity or makes me a better person, but for the last few years I've generally taken at least a half hour out of the middle of my workday to walk (I'm lucky enough to work next to a park). It massively improves my mood, and I'd fight hard against giving it up.

There was a tube (underground metro) strike in London the past few days, so I walked to work, leaving my apartment at about 6.30am and arriving in the office at 8am, mostly along the river. It was excellent and I'm considering doing it 1-2 days a week even when I don't need to.

throwaway13qf85

12 anni fadiscuss

I'm really excited by Julia. We use Matlab heavily in my team (as in, we probably have 300-600 Matlab instances running at any given time not counting the ones on personal workstations) and I'd love to use a language with similar support for numerical programming and linear algebra, but with better typing and functional programming support.

Do you have any suggestions for slowly introducing Julia to the team?

throwaway13qf85

12 anni fadiscuss

It's tough for someone who's not a professional musician to tell the difference between a just fifth (3:2) and an equal tempered fifth (2^(7/12):1) but it's not impossible.

Probably the easiest way is to go to a piano and play a fifth with the base note at middle C. The sound noticeably changes after a couple of seconds. That's because the ratio isn't exactly 3:2 and the frequencies of the pure notes start to get out of alignment (the same phenomenon causes 'beats' when two similar but not exactly equal frequencies are played together).

It's even more pronounced when you play a C major chord (C-E-G) because the major third (C-E) isn't exactly 5:4 and the minor third (E-G) isn't exactly 6:5 either.