221 has 4 divisors (see below), whose sum is σ = 252. Its totient is φ = 192.

The previous prime is 211. The next prime is 223. The reversal of 221 is 122.

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 can be written as a sum of positive squares in 2 ways, for example, as 100 + 121 = 10^2 + 11^2 .

It is a cyclic number.

It is not a de Polignac number, because 221 - 2^{6} = 157 is a prime.

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.

It is not an unprimeable number, because it can be changed into a prime (223) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 5 + ... + 21.

It is an arithmetic number, because the mean of its divisors is an integer number (63).

221 is the 11-th centered square number.

It is an amenable number.

221 is a deficient number, since it is larger than the sum of its proper divisors (31).

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 product of its digits is 4, while the sum is 5.

The square root of 221 is about 14.8660687473. The cubic root of 221 is about 6.0459435960.

The spelling of 221 in words is "two hundred twenty-one", and is thus an aban number and an iban number.

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