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More Undecidable Problems

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Book cover Limits of Computation

Part of the book series: Undergraduate Topics in Computer Science ((UTICS))

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Abstract

In this chapter we will look at more undecidable problems. We show a famous result, Rices theorem, that any non-trivial purely semantical property of programs undecidable. The proof uses a reduction from the Halting problem. Such reductions and the reasoning principles they give rise to are investigated. The concept of a semi-decidable problem is introduced, and it is shown that the Halting problem is semi-decidable by means of the self-interpreter.

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Notes

  1. 1.

    Henry Gordon Rice (July 18, 1920–April 14, 2003) was an American logician and mathematician. He proved “his” theorem in his doctoral dissertation of 1951.

  2. 2.

    Program verification is actually a thriving discipline and quite considerable progress has been made in the last few decades [21] and has even reached giants like Facebook [4].

  3. 3.

    Wang (20 May 1921–13 May 1995) was born in China but later taught at Harvard where he was also the thesis advisor of Stephen Cook (of whom we will hear more later).

  4. 4.

    Problem reduction is a strategy that human beings or companies use all the time in an informal manner. For instance to “solve” the problem of getting from home to work, one could reduce it for instance to the problem of getting to the train station and on the right train, provided one already has a ticket.

  5. 5.

    That means that for different \(x\ne y\in A\) we can still have \(f(x)=f(y)\).

  6. 6.

    More precisely, “recursive” refers to the class of “partial recursive functions” on numbers (as introduced by Kleene in the late 1930s) which one can show are also a reasonable notion of computation equivalent to all the one we have presented.

  7. 7.

    Again, note that this does not mean we cannot write a program that might produce the answer for a specific input value. If this input is finite then a brute force search will always work. For instance, we can clearly write a program that decides whether a floor of a given fixed size (say \(5\times 5\)) can be tiled with given, say k, tile types since there are \(k^{25}\) possible ways to tile such a floor. Note that there is a combinatorial explosion of the number of possible tilings but this does not concern us in computability. It will concern us in the part about complexity.

  8. 8.

    Haskell and ML are such programming languages but they exclude the type of all types.

  9. 9.

    It may also return false positives, i.e. accept programs that are not type sound, this happens e.g. in the case of division-by-zero.

  10. 10.

    For a good textbook on types see [16].

  11. 11.

    The characteristic function \(\chi \) of a set A of elements of type T is of type \(T\rightarrow \mathbb {B}\) and defined by \(\chi (d) = d\in A\).

  12. 12.

    Tibor Radó (June 2, 1895–December 29, 1965) was a Hungarian mathematician who moved to the USA and worked in Computer Science later in his life. The publication on the Busy Beaver function is maybe his most famous one [19].

  13. 13.

    This function is named after Wilhelm Friedrich Ackermann (29 March 1896–24 December 1962), a German mathematician, and Rózsa Péter, born Politzer, (17 February 1905–16 February 1977), a Hungarian mathematician.

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Reus, B. (2016). More Undecidable Problems. In: Limits of Computation. Undergraduate Topics in Computer Science. Springer, Cham. https://doi.org/10.1007/978-3-319-27889-6_9

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  • DOI: https://doi.org/10.1007/978-3-319-27889-6_9

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