21387645 has 48 divisors (see below), whose sum is σ = 39836160. Its totient is φ = 10995264.

The previous prime is 21387643. The next prime is 21387661. The reversal of 21387645 is 54678312.

21387645 is a `hidden beast` number, since 2 + 1 + 3 + 8 + 7 + 645 = 666.

It is not a de Polignac number, because 21387645 - 2¹ = 21387643 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21387643) by changing a digit.

It is a polite number, since it can be written in 47 ways as a sum of consecutive naturals, for example, 34932 + ... + 35538.

It is an arithmetic number, because the mean of its divisors is an integer number (829920).

Almost surely, 2^21387645 is an apocalyptic number.

It is an amenable number.

21387645 is a deficient number, since it is larger than the sum of its proper divisors (18448515).

21387645 is an equidigital number, since it uses as much as digits as its factorization.

21387645 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 656 (or 644 counting only the distinct ones).

The product of its digits is 40320, while the sum is 36.

The square root of 21387645 is about 4624.6778266167. The cubic root of 21387645 is about 277.5796703312.

The spelling of 21387645 in words is "twenty-one million, three hundred eighty-seven thousand, six hundred forty-five".