21210821 has 8 divisors (see below), whose sum is σ = 22372320. Its totient is φ = 20053872.

The previous prime is 21210799. The next prime is 21210823. The reversal of 21210821 is 12801212.

It is a happy number.

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 21210821 - 2¹⁸ = 20948677 is a prime.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 21210821.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21210823) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 13166 + ... + 14688.

It is an arithmetic number, because the mean of its divisors is an integer number (2796540).

Almost surely, 2^21210821 is an apocalyptic number.

It is an amenable number.

21210821 is a deficient number, since it is larger than the sum of its proper divisors (1161499).

21210821 is a wasteful number, since it uses less digits than its factorization.

21210821 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2275.

The product of its (nonzero) digits is 64, while the sum is 17.

The square root of 21210821 is about 4605.5207088884. The cubic root of 21210821 is about 276.8125820194.

The spelling of 21210821 in words is "twenty-one million, two hundred ten thousand, eight hundred twenty-one".