21253721 has 2 divisors, whose sum is σ = 21253722. Its totient is φ = 21253720.

The previous prime is 21253711. The next prime is 21253759. The reversal of 21253721 is 12735212.

It is a happy number.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 18156121 + 3097600 = 4261^2 + 1760^2 .

It is a cyclic number.

It is not a de Polignac number, because 21253721 - 2⁶ = 21253657 is a prime.

It is not a weakly prime, because it can be changed into another prime (21253711) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10626860 + 10626861.

It is an arithmetic number, because the mean of its divisors is an integer number (10626861).

Almost surely, 2^21253721 is an apocalyptic number.

It is an amenable number.

21253721 is a deficient number, since it is larger than the sum of its proper divisors (1).

21253721 is an equidigital number, since it uses as much as digits as its factorization.

21253721 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 840, while the sum is 23.

The square root of 21253721 is about 4610.1758100966. The cubic root of 21253721 is about 276.9990790066. Note that the first 3 decimals are identical.

Adding to 21253721 its reverse (12735212), we get a palindrome (33988933).

The spelling of 21253721 in words is "twenty-one million, two hundred fifty-three thousand, seven hundred twenty-one".