21657840 has 160 divisors (see below), whose sum is σ = 71995392. Its totient is φ = 5376000.

The previous prime is 21657833. The next prime is 21657863. The reversal of 21657840 is 4875612.

It is a Smith number, since the sum of its digits (33) coincides with the sum of the digits of its prime factors.

It is a zygodrome in base 9.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 305005 + ... + 305075.

Almost surely, 2^21657840 is an apocalyptic number.

21657840 is a gapful number since it is divisible by the number (20) 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 21657840, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (35997696).

21657840 is an abundant number, since it is smaller than the sum of its proper divisors (50337552).

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

21657840 is a wasteful number, since it uses less digits than its factorization.

21657840 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 159 (or 153 counting only the distinct ones).

The product of its (nonzero) digits is 13440, while the sum is 33.

The square root of 21657840 is about 4653.7984485794. The cubic root of 21657840 is about 278.7436913317.

The spelling of 21657840 in words is "twenty-one million, six hundred fifty-seven thousand, eight hundred forty".