5
This number is a prime.
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The only prime member of a Ruth-Aaron pair. [Caldwell]
The sum of the reciprocals of the primes is infinite, but
the sum of the reciprocals of the known primes is less than
5 and will always be so! [Caldwell]
Every positive integer can be written as
x2+2y2+7z2+11w2, except for 5. [Halmos1938] [Caldwell]
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