21640256 has 56 divisors (see below), whose sum is σ = 47731680. Its totient is φ = 9651200.

The previous prime is 21640249. The next prime is 21640273. The reversal of 21640256 is 65204612.

It is a hoax number, since the sum of its digits (26) coincides with the sum of the digits of its distinct prime factors.

It is an unprimeable number.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 41276 + ... + 41796.

Almost surely, 2^21640256 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 21640256, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (23865840).

21640256 is an abundant number, since it is smaller than the sum of its proper divisors (26091424).

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

21640256 is a wasteful number, since it uses less digits than its factorization.

21640256 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 603 (or 593 counting only the distinct ones).

The product of its (nonzero) digits is 2880, while the sum is 26.

The square root of 21640256 is about 4651.9088555130. The cubic root of 21640256 is about 278.6682335736.

Adding to 21640256 its reverse (65204612), we get a palindrome (86844868).

The spelling of 21640256 in words is "twenty-one million, six hundred forty thousand, two hundred fifty-six".