Re: Can EXPTIME and NP be separated via diagonalization?



In article <59180830-935b-495a-b3f4-6b007b117e51@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
<cplxphil@xxxxxxxxx> wrote:
If my argument is correct, this means that there is no algorithm that
decides membership of a language in NP.

It's not clear what exactly you mean by the problem of deciding whether a
given language is in NP. A language is (usually) an infinite set, so you
can't provide it directly as input to a computer program. I'm guessing
that you're representing the language via some kind of *finite description*
of the language. But what kind of finite description do you have in mind?
An arbitrary Turing machine? But we know by Rice's theorem that we can't
determine *any* nontrivial property of the language accepted by a given
machine, let alone the property of being in NP. So maybe you have some
other finite description in mind?
--
Tim Chow tchow-at-alum-dot-mit-dot-edu
The range of our projectiles---even ... the artillery---however great, will
never exceed four of those miles of which as many thousand separate us from
the center of the earth. ---Galileo, Dialogues Concerning Two New Sciences
.



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