21156221 has 4 divisors (see below), whose sum is σ = 21291132. Its totient is φ = 21021312.

The previous prime is 21156181. The next prime is 21156227. The reversal of 21156221 is 12265112.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 15912121 + 5244100 = 3989^2 + 2290^2 .

It is a cyclic number.

It is not a de Polignac number, because 21156221 - 2¹⁸ = 20894077 is a prime.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 21156193 and 21156202.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21156227) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 67220 + ... + 67533.

It is an arithmetic number, because the mean of its divisors is an integer number (5322783).

Almost surely, 2^21156221 is an apocalyptic number.

It is an amenable number.

21156221 is a deficient number, since it is larger than the sum of its proper divisors (134911).

21156221 is a wasteful number, since it uses less digits than its factorization.

21156221 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 134910.

The product of its digits is 240, while the sum is 20.

The square root of 21156221 is about 4599.5892207892. The cubic root of 21156221 is about 276.5748581687.

It can be divided in two parts, 21 and 156221, that multiplied together give a triangular number (3280641 = T₂₅₆₁).

The spelling of 21156221 in words is "twenty-one million, one hundred fifty-six thousand, two hundred twenty-one".