221

This number is a composite.

2*3*5*7+11=13*17. Note the consecutive use of the first 7 primes. [Rupinski]

221^221 + reversal(221) is prime. It is the only known number greater than 1 with this property. [Firoozbakht]

The two cicada broods, 13-year cicada and the 17-year cicada (whose development times has curious connections to prime numbers), emerge together once in 221 years. It happened in 2024 and would next happen in 2245. [Gudipati]

The smallest squarefree brilliant number that represents the hypotenuse of four Pythagoren triangles, i.e., (21^2+220^2=221^2), (85^2+204^2=221^2), (104^2+195^2=221^2), (140^2+171^2=221^2). Curiously, 221 is also expressible as the sum of two squares in two different ways: (5^2+14^2 = 221 = 10^2+11^2). [Bajpai]