21720021 has 16 divisors (see below), whose sum is σ = 30858240. Its totient is φ = 13549680.

The previous prime is 21720007. The next prime is 21720047. The reversal of 21720021 is 12002712.

It is not a de Polignac number, because 21720021 - 2⁵ = 21719989 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21720121) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 2436 + ... + 7026.

It is an arithmetic number, because the mean of its divisors is an integer number (1928640).

Almost surely, 2^21720021 is an apocalyptic number.

It is an amenable number.

21720021 is a deficient number, since it is larger than the sum of its proper divisors (9138219).

21720021 is a wasteful number, since it uses less digits than its factorization.

21720021 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4696.

The product of its (nonzero) digits is 56, while the sum is 15.

The square root of 21720021 is about 4660.4743320825. The cubic root of 21720021 is about 279.0101999072.

Adding to 21720021 its reverse (12002712), we get a palindrome (33722733).

The spelling of 21720021 in words is "twenty-one million, seven hundred twenty thousand, twenty-one".