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I struggled to understand the Cantor's diagonal argument, but I have some problems comprehending the following:

By construction, $s$ differs from each $s_n$, since their $n^{th}$ digits differ (highlighted in the example). Hence, $s$ cannot occur in the enumeration.

Can someone explain why the built sequence $s$ cannot occur in the enumeration? I can imagine it somehow recursively that if we add it to $s_n$, we can pick another sequence with the same construction pattern and we can repeat these steps forever. However, I want to know if this is sufficient as a proof or there is some kind of other explanation.

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closed as off-topic by Tsuyoshi Ito, Sasho Nikolov, Radu GRIGore, R B, Kaveh Jan 17 '15 at 10:14

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It's actually more trivial.

  1. Suppose that $s$ occurs in the enumeration.

  2. Then it occurs at some specific index. Let's call this index $n$. This means that $s = s_n$.

  3. But this is impossible, because $s$ and $s_n$ differ in the $n$th digit.

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