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**1 - 1**of**1**### Chapter 0: The Easy Way to Gödel’s Proof and Related Topics ∗

"... This short sketch of Gödel’s incompleteness proof shows how it arises naturally from Cantor’s diagonalization method [1891]. It renders Gödel’s proof and its relation to the semantic paradoxes transparent. Some historical details, which are often ignored, are pointed out. We also make some observati ..."

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This short sketch of Gödel’s incompleteness proof shows how it arises naturally from Cantor’s diagonalization method [1891]. It renders Gödel’s proof and its relation to the semantic paradoxes transparent. Some historical details, which are often ignored, are pointed out. We also make some observations on circularity and draw brief comparisons with natural language. The sketch does not include the messy details of the arithmetization of the language, but the motives for it are made obvious. We suggest this as a more efficient way to teach the topic than what is found in the standard textbooks. For the sake of self–containment Cantor’s original diagonalization is included. A broader and more technical perspective on diagonalization is given in [Gaifman 2005]. In [1891] Cantor presented a new type of argument that shows that the set of all binary sequences (sequences of the form a0, a1,…,an,…, where each ai is either 0 or 1) is not denumerable ─ that is, cannot be arranged in a sequence, where the index ranges over the natural numbers. Let A0, A2,…An, … be a sequence of binary sequences. Say An = an,0, an,1, …, an,i, …. Define a new sequence A * = b0, b1,…,bn, … , by putting: bn = 1, if an,n = 0, bn = 0, if an,n = 1 Then, for each n, A * ≠ An, since the n th member of A * differs from the the n th member of An. Hence, A * does not appear among the Ai’s. A diagram of the following form, which appears already in Cantor’s original paper, illustrate the idea. The new sequence A * is obtained from the diagonal, by changing each of its values. The method came to be known as diagonalization. A0 = a0,0 a0,1... a0,n... A1 = a1,0 a1,1... a1,n...