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# THEOREM OF THE DAY

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## THEOREM OF THE DAY

### G¨odel’s First Incompleteness Theorem There is no consistent and complete, recursively enumerableaxiomatisation of number theory. That is, any such axiomatisation will either yield a proof for some falsestatement or will fail to yield a proof for some true one.

“This sentence is false!” Kurt G¨odel had a genius for turning such philosophical paradoxes into formal mathematics. In a recursively enumerable axiomatisation, T , all sentences — statements and proofs of statements — can, in principle, be listed systematically, although this enumeration will never end, since the list is infinite. This idea was captured by G¨odel by giving each sentence s a unique number, denotedpsq and now called a G¨odel number, a product of powers of primes. On the right of the picture, pay particular attention to the number 384.534. . . .2335.2966.This is a number over five hundred digits long — never mind! It will be taken to represent the first-order predicate on the left: ∀x¬P(x,y), “for all x, P(x,y) is false,” which we will denote G(y). Next, G¨odel proved a fixed point result: for any arithmetic predicate G, we can find a number g so that the G¨odel number of G with g as input is again the same number: pG(g)q = g.

Now suppose that P(x,y) is actually the two-valued predicate which is true if and only if x is the G¨odel number of a sentence proving statement number y. Then G(g) means: “sentence number g has no proof in our numbering system”. Suppose G(g) is provable within T , which is the same as saying that sentence number g has a proof. But this reveals G(g) to be false, and producing a proof of a falsehood is precisely what is meant by saying that T is not consistent. So now if T is consistent we therefore know that G(g) cannot be provable, in other words, sentence number g has no proof — G(g) is true! Conclusion: G(g) is a true statement but one which has no proof.

### G¨odel’s announcement of this theorem, in 1931, instantly and forever banished the notion of mathematics as a complete and infallible body of knowledge; and in particular refuted the efforts of Frege, Hilbert, Russell and others to redefine mathematics as a self-contained system of formal logic.

Further reading:An Introduction to G¨odel’s Theoremsby Peter Smith, Cambridge University Press, 2007.

Theorem of the Day is maintained by Robin Whitty atwww.theoremoftheday.org

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