21209821 has 2 divisors, whose sum is σ = 21209822. Its totient is φ = 21209820.

The previous prime is 21209813. The next prime is 21209869. The reversal of 21209821 is 12890212.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 18020025 + 3189796 = 4245^2 + 1786^2 .

It is a cyclic number.

It is not a de Polignac number, because 21209821 - 2³ = 21209813 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21209801) 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 as a sum of consecutive naturals, namely, 10604910 + 10604911.

It is an arithmetic number, because the mean of its divisors is an integer number (10604911).

Almost surely, 2^21209821 is an apocalyptic number.

It is an amenable number.

21209821 is a deficient number, since it is larger than the sum of its proper divisors (1).

21209821 is an equidigital number, since it uses as much as digits as its factorization.

21209821 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 576, while the sum is 25.

The square root of 21209821 is about 4605.4121422518. The cubic root of 21209821 is about 276.8082317724.

The spelling of 21209821 in words is "twenty-one million, two hundred nine thousand, eight hundred twenty-one".