73

This number is a prime.

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There are exactly 73 primes, beginning with the prime 1093 and ending with the prime 1613, where 1093² + 1097² + ... + 1613² = 11707². This is the first instance of a prime number of primes comprising the left member of such an equation. [Haga]

The smallest prime whose digits are reversed in base 22. [Haga]

The content editor of "Prime Curios!" is credited on page 73 of R. Crandall and C. Pomerance, Prime Numbers: A Computational Perspective, Springer-Verlag, NY, (2002 printing), with noting that 61 divides 67*71 + 1. [Haga]

There are 73 composite numbers under 10^2. Proof: 73+25 primes = 98. There are 99 numbers under 10^2, but 1 is neither prime nor composite. This is the only instance of a prime count of composites through 10^22. [Haga]