9

This number is a composite.

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The smallest semiprime such that all permutations of concatenations with its factors are semiprimes. [Sariyar]

Regular nonagon is the only regular polygon such that n^n +(n-2)*180/n (387420629) and ((n-2)*180/n)^n+n (20661046784000000009) are both prime. Note that (n-2)*180/n is an internal angel of a regular polygon. [Sariyar]

Only 9 numbers have been found that are the sums of two distinct primes in exactly two ways. [Sariyar]