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New Theories / Re: What makes NP problems hard to solve?
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Donald Knuth: P=NP | AI Podcast Clips
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Donald Knuth is one of the greatest and most impactful computer scientists and mathematicians ever. He is the recipient in 1974 of the Turing Award, considered the Nobel Prize of computing. He is the author of the multi-volume work, the magnum opus, The Art of Computer Programming. He made several key contributions to the rigorous analysis of the computational complexity of algorithms. He popularized asymptotic notation, that we all affectionately know as the big-O notation. He also created the TeX typesetting which most computer scientists, physicists, mathematicians, and scientists and engineers use to write technical papers and make them look beautiful.
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He is referring to the Robertson & Seymour graph minor theorem. Every class of graphs that is closed under minors has a finite obstruction set of "forbidden minors". This follows from the fact that the graph minor relation on the class of all graphs is a quasi-well ordering.
You can look it up on wikipedia.
The point is that this theorem constitutes a non-constructive proof for the existence of PTIME algorithms for every class of minor closed graphs. I.e. we know that a PTIME algorithm EXIST for each these classes, but we do not know what exactly the PTIME algorithm IS for each individual class, because for this you would first need to know the finite obstruction set of forbidden minors.
So this theorem contradicts the intuition that the only way to find out whether a problem is in P is to write down an explicit algorithm for solving it. You can know that a problm is in P without knowing what the PTIME algorithm for solving it is. And this might suggest that you really can't conclude that P!=NP just from the fact that no one has found a PTIME algorrithm for a NP-hard problem yet.