21

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+ 21 repeated twenty-one times, following 1, forms a smoothly undulating palindromic prime. [Silva]

+ 21 followed by its reversal, plus/minus one, are twin primes. The first integer with that property. [Silva]

+ The first Fibonacci semiprime is followed by two others (34 and 55). [Silva]

(There are 2 curios for this number that have not yet been approved by an editor.)

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