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Whilst re-posting a review of Douglas

Hofstadter’s Gödel, Escher, Bach: An Eternal Golden Braid, I visited a computability topic long been of interest - The P=NP question and Gödel’s famous 1956 letter to John von Neumann (click the image for the hand written german text)

The P=NP question is one of the great problems of science, which has intrigued computer scientists and mathematicians for decades. In the first half of this century, work on the power of formal systems led to the formalization of the notion of algorithm and the realization that certain problems are algorithmically unsolvable.

NP stands for Non-deterministic Polynomial time meaning that a problem can be solved in Polynomial time using a Non-deterministic Turing machine (like a regular Turing machine but also including a non-deterministic "choice" function). Basically, a solution has to be *testable* in poly time. If that's the case, and a known NP problem can be solved using the given problem with modified input (an NP problem can be *reduced* to the given problem) then the problem is NP complete.

The main thing to take away from an NP-complete problem is that it cannot be solved in polynomial time in any known way. NP-Hard/NP-Complete is a way of showing that certain classes of problems are not solvable in realistic time.

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# The Gödel Letter1

Princeton, 20 March 1956

Dear Mr. von Neumann:

With the greatest sorrow I have learned of your illness. The news came to me as quite unexpected. Morgenstern already last summer told me of a bout of weakness you once had, but at that time he thought that this was not of any greater significance. As I hear, in the last months you have undergone a radical treatment and I am happy that this treatment was successful as desired, and that you are now doing better. I hope and wish for you that your condition will soon improve even more and that the newest medical discoveries, if possible, will lead to a complete recovery.

Since you now, as I hear, are feeling stronger, I would like to allow myself to write you about a mathematical problem, of which your opinion would very much interest me: One can obviously easily construct a Turing machine, which for every formula F in first order predicate logic and every natural number n, allows one to decide if there is a proof of F of length n (length = number of symbols).

Let ψ(F,n) be the number of steps the machine requires for this and let φ(n) = maxF ψ(F,n). The question is how fast φ(n) grows for an optimal machine. One can show that φ(n) ≥ k ⋅ n. If there really were a machine with φ(n) ∼ k ⋅ n (or even ∼ k ⋅ n^{2}), this would have consequences of the greatest importance. Namely, it would obviously mean that in spite of the undecidability of the Entscheidungs problem, the mental work of a mathematician concerning Yes-or-No questions could be completely replaced by a machine. After all, one would simply have to choose the natural number n so large that when the machine does not deliver a result, it makes no sense to think more about the problem. Now it seems to me, however, to be completely within the realm of possibility that φ(n) grows that slowly. Since it seems that φ(n) ≥ k ⋅ n is the only estimation which one can obtain by a generalization of the proof of the undecidability of the Entscheidungs problem and after all φ(n) ∼ k ⋅ n (or ∼ k ⋅ n^{2}) only means that the number of steps as opposed to trial and error can be reduced from N to log N (or (log N)^{2}).

However, such strong reductions appear in other finite problems, for example in the computation of the quadratic residue symbol using repeated application of the law of reciprocity. It would be interesting to know, for instance, the situation concerning the determination of primality of a number and how strongly in general the number of steps in finite combinatorial problems can be reduced with respect to simple exhaustive search.

I do not know if you have heard that “Post’s problem”, whether there are degrees of unsolvability among problems of the form (∃ y) φ(y,x), where φ is recursive, has been solved in the positive sense by a very young man by the name of Richard Friedberg. The solution is very elegant. Unfortunately, Friedberg does not intend to study mathematics, but rather medicine (apparently under the influence of his father). By the way, what do you think of the attempts to build the foundations of analysis on ramified type theory, which have recently gained momentum? You are probably aware that Paul Lorenzen has pushed ahead with this approach to the theory of Lebesgue measure. However, I believe that in important parts of analysis non-eliminable impredicative proof methods do appear.

I would be very happy to hear something from you personally. Please let me know if there is something that I can do for you. With my best greetings and wishes, as well to your wife,

Your very devoted,

Kurt Gödel

P.S. I congratulate you heartily on the … 2

1. The letter was indeed originally written in German. Mike Sipser’s translation can be found in “The History and the Status of the P versus NP Question”, in the 24th STOC proceedings, 1992, pp. 603-618. This also contains a number of other historical quotes. Peter Clote also made a translation to English, which can be found in the book “Arithmetic, Proof Theory and Computational Complexity”, P. Clote and J. Krajicek, eds., Oxford Univ. Press, 1993.

2. The balance of the postscript is missing.