21720215 has 16 divisors (see below), whose sum is σ = 28524960. Its totient is φ = 15745920.

The previous prime is 21720197. The next prime is 21720229. The reversal of 21720215 is 51202712.

It is not a de Polignac number, because 21720215 - 2⁸ = 21719959 is a prime.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 30281 + ... + 30989.

It is an arithmetic number, because the mean of its divisors is an integer number (1782810).

Almost surely, 2^21720215 is an apocalyptic number.

21720215 is a deficient number, since it is larger than the sum of its proper divisors (6804745).

21720215 is a wasteful number, since it uses less digits than its factorization.

21720215 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1282.

The product of its (nonzero) digits is 280, while the sum is 20.

The square root of 21720215 is about 4660.4951453681. The cubic root of 21720215 is about 279.0110305972.

Adding to 21720215 its reverse (51202712), we get a palindrome (72922927).

The spelling of 21720215 in words is "twenty-one million, seven hundred twenty thousand, two hundred fifteen".