21640788 has 54 divisors (see below), whose sum is σ = 59251374. Its totient is φ = 6656832.

The previous prime is 21640783. The next prime is 21640789. The reversal of 21640788 is 88704612.

21640788 is a `hidden beast` number, since 2 + 1 + 640 + 7 + 8 + 8 = 666.

It can be written as a sum of positive squares in 3 ways, for example, as 956484 + 20684304 = 978^2 + 4548^2 .

It is a Harshad number since it is a multiple of its sum of digits (36).

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21640783) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 17 ways as a sum of consecutive naturals, for example, 4306 + ... + 7862.

Almost surely, 2^21640788 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 21640788, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (29625687).

21640788 is an abundant number, since it is smaller than the sum of its proper divisors (37610586).

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

21640788 is a wasteful number, since it uses less digits than its factorization.

21640788 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3593 (or 3575 counting only the distinct ones).

The product of its (nonzero) digits is 21504, while the sum is 36.

The square root of 21640788 is about 4651.9660359895. The cubic root of 21640788 is about 278.6705171307.

The spelling of 21640788 in words is "twenty-one million, six hundred forty thousand, seven hundred eighty-eight".