21251664 has 120 divisors (see below), whose sum is σ = 70412160. Its totient is φ = 5854464.

The previous prime is 21251617. The next prime is 21251669. The reversal of 21251664 is 46615212.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21251669) by changing a digit.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 28869 + ... + 29595.

It is an arithmetic number, because the mean of its divisors is an integer number (586768).

Almost surely, 2^21251664 is an apocalyptic number.

21251664 is a gapful number since it is divisible by the number (24) formed by its first and last digit.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21251664, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (35206080).

21251664 is an abundant number, since it is smaller than the sum of its proper divisors (49160496).

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

21251664 is a wasteful number, since it uses less digits than its factorization.

21251664 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 777 (or 768 counting only the distinct ones).

The product of its digits is 2880, while the sum is 27.

The square root of 21251664 is about 4609.9527112542. The cubic root of 21251664 is about 276.9901424461.

Adding to 21251664 its reverse (46615212), we get a palindrome (67866876).

The spelling of 21251664 in words is "twenty-one million, two hundred fifty-one thousand, six hundred sixty-four".