JoshuaZ comments on The Curve of Capability - Less Wrong

18 Post author: rwallace 04 November 2010 08:22PM

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Comment author: soreff 06 November 2010 06:50:23PM *  2 points [-]

I'm confused or about to display my ignorance. When you write:

There are many fairly natural problems which are not in P. For example, given a specific Go position does white or black have a win? This is in EXP

Are you saying that evaluating a Go position has exponential (time?) cost even on a nondeterministic machine? I thought that any given string of moves to the end of the game could be evaluated in polynomial (N^2?) time. I thought that the full set of possible strings of moves could be evaluated (naively) in exponential time on a deterministic machine and polynomial time on a nondeterministic machine - so I thought Go position evaluation would be in NP. I think you are saying that it is more costly than that. Are you saying that, and what am I getting wrong?

Comment author: JoshuaZ 08 November 2010 03:29:00AM 0 points [-]

I'm not completely sure what you mean. Darius's response seems relevant (in particular, you may want to look at the difference between a general non-deterministic Turing machine and an alternating Turing machine). However, there seems to be possibly another issue here: When mathematicians and computer scientists discuss polynomial time, they are talking about polynomials of the length of the input, not polynomials of the input (similarly, for exponential time and other classes). Thus, for example, to say that PRIMES is in P we mean that there's an algorithm that answers whether a given integer is prime that is time bounded by a polynomial of log_2 p (assuming p is written in base 2).