21720765 has 16 divisors (see below), whose sum is σ = 37912896. Its totient is φ = 10531200.

The previous prime is 21720749. The next prime is 21720779. The reversal of 21720765 is 56702712.

It is not a de Polignac number, because 21720765 - 2⁴ = 21720749 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, 65656 + ... + 65985.

It is an arithmetic number, because the mean of its divisors is an integer number (2369556).

Almost surely, 2^21720765 is an apocalyptic number.

It is an amenable number.

21720765 is a deficient number, since it is larger than the sum of its proper divisors (16192131).

21720765 is a wasteful number, since it uses less digits than its factorization.

21720765 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 131660.

The product of its (nonzero) digits is 5880, while the sum is 30.

The square root of 21720765 is about 4660.5541516004. The cubic root of 21720765 is about 279.0133856195.

The spelling of 21720765 in words is "twenty-one million, seven hundred twenty thousand, seven hundred sixty-five".