21003664 has 60 divisors (see below), whose sum is σ = 47167120. Its totient is φ = 9028800.

The previous prime is 21003659. The next prime is 21003667. The reversal of 21003664 is 46630012.

It is a Harshad number since it is a multiple of its sum of digits (22).

It is not an unprimeable number, because it can be changed into a prime (21003667) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 36499 + ... + 37069.

Almost surely, 2^21003664 is an apocalyptic number.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21003664, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (23583560).

21003664 is an abundant number, since it is smaller than the sum of its proper divisors (26163456).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

21003664 is a wasteful number, since it uses less digits than its factorization.

21003664 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 620 (or 603 counting only the distinct ones).

The product of its (nonzero) digits is 864, while the sum is 22.

The square root of 21003664 is about 4582.9754526945. The cubic root of 21003664 is about 275.9084622577.

Adding to 21003664 its reverse (46630012), we get a palindrome (67633676).

The spelling of 21003664 in words is "twenty-one million, three thousand, six hundred sixty-four".