Surprising Math

1. A pleasant surprise

When I worked on the diameter problem for d-polytopes with n facets. I was aiming to prove an upper  bound of the form n^{\log d} but my proof only gave d^{\log n},

It was a pleasant surprise to note that n^{\log d}=d^{\log n}.

2. A bigger surprise

A few weeks ago James Lee gave a talk and proved a bound of the form (\log n)^{\log \log \log n}.   I was surprised to learn from him that (\log n)^{\log \log \log n} = ( \log \log n)^{\log \log n} .

(Update: I got it wrong at first, thanks guys)

This is an even more surprising special case of the formula above.

3.  Is it better to have the discount first?

Question: What is a better deal: A store that gives 12% student-discount after it adds a 12% value added tax to the price of the product? Or a store that first adds 12% tax on the entire sum and only then deducts 12% student discount?

Ohh, The way I asked this question the two alternatives are precisely the same. Let me ask it again: What is a better deal: A store that gives 12% student discount after adding a 12% value added tax to the price of the product? Or a store that first deducts the 12% student discount, and only then adds 12% tax on the new price? 

Answer: The same, by a surprisingly not obvious special case of commutativity of multiplication.

(See a related comment on Dave Bacon blog.)

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4 Responses to Surprising Math

  1. RandomWalker says:

    Ok, so point (1) is explained by n^log d = e ^ ((log n) (log d)) = d ^ log n

    But then, similarly for (2), how is
    (log log log log n) * (log n) = (log log log n) * (log log n) ?

    Am I missing something?

  2. Radu Grigore says:

    I would guess it should be Log[Log[n]]^(Log[Log[n]])=Log[n]^(Log[Log[Log[n]]])

  3. roland says:

    What is not obvious about the third point?

  4. Gil says:

    Roland, I suspect this is much less obvious to most people than other cases regarding multiplication being commutative. This can be examined empirically, of course. (At the time, I found it confusing myself, but this may not tell much .)

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