21720387 has 12 divisors (see below), whose sum is σ = 31360344. Its totient is φ = 13366080.

The previous prime is 21720371. The next prime is 21720389. The reversal of 21720387 is 78302712.

It is not a de Polignac number, because 21720387 - 2⁴ = 21720371 is a prime.

It is a super-2 number, since 2×21720387² = 943550422859538, which contains 22 as substring.

It is a Smith number, since the sum of its digits (30) coincides with the sum of the digits of its prime factors.

It is not an unprimeable number, because it can be changed into a prime (21720389) 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 in 11 ways as a sum of consecutive naturals, for example, 20914 + ... + 21927.

It is an arithmetic number, because the mean of its divisors is an integer number (2613362).

Almost surely, 2^21720387 is an apocalyptic number.

21720387 is a deficient number, since it is larger than the sum of its proper divisors (9639957).

21720387 is a wasteful number, since it uses less digits than its factorization.

21720387 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 42870 (or 42857 counting only the distinct ones).

The product of its (nonzero) digits is 4704, while the sum is 30.

The square root of 21720387 is about 4660.5135983065. The cubic root of 21720387 is about 279.0117670812.

The spelling of 21720387 in words is "twenty-one million, seven hundred twenty thousand, three hundred eighty-seven".