221 is nontrivially palindromic in base 7, base 11 and base 16.
221 is an esthetic number in base 7, because in such base it adjacent digits differ by 1.
It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length, and also an emirpimes, since its reverse is a distinct semiprime: 122 = 2 ⋅61.
It is a cyclic number.
It is a magnanimous number.
It is an Ulam number.
It is a Duffinian number.
221 is an undulating number in base 4, base 7 and base 11.
It is a Curzon number.
221 is a nontrivial repdigit in base 16.
It is a plaindrome in base 8, base 14 and base 16.
It is a nialpdrome in base 10, base 15 and base 16.
It is a zygodrome in base 16.
It is a congruent number.
221 is the 11-th centered square number.
It is an amenable number.
221 is a wasteful number, since it uses less digits than its factorization.
221 is an evil number, because the sum of its binary digits is even.
The sum of its prime factors is 30.
The square root of 221 is about 14.8660687473. The cubic root of 221 is about 6.0459435960.