23

This number is a prime.

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Kristen Eller ©2002

How many terms of the series 1 + 1/2 + 1/3 + ... + 1/9 + 1/11 + ... (which excludes all denominators containing 0) is required to produce a sum greater than 23? [Honaker]

23 is the smallest prime that has a number of composites before and after it that form a twin prime pair, i.e., 19 (3 composites) 23 (5 composites) 29 and {3, 5} is a set of twin primes. [Honaker]