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This number is a composite.
Just showing those entries submitted by 'Silva':
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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]
The 21st odd prime is the sum of first semiprimes up to 21. [Silva]
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