In CS131, we see the “Proof from the Book” due to Euclide that the number of prime numbers is infinite. Here is another favorite proof that is based on elementary facts. Suppose that is a list of all the primes. From what we have learnt about the sum of geometric progressions we know that for **any** :

Multiplying these inequalities term by term, we obtain:

Observe the following.

- The LHS of the inequality is strictly greater than .

By combining the above, we derive the conclusion that for any n, the RHS is greater than . Contradiction, since assuming that we have a finite number of primes, the RHS should be finite. Therefore, the set of prime numbers is infinite.

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