21

This number is a composite.

Just showing those entries submitted by 'Silva': (Click here to show all)

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]