21665800 has 72 divisors (see below), whose sum is σ = 54630990. Its totient is φ = 7987200.

The previous prime is 21665789. The next prime is 21665827. The reversal of 21665800 is 856612.

It can be written as a sum of positive squares in 9 ways, for example, as 191844 + 21473956 = 438^2 + 4634^2 .

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 17 ways as a sum of consecutive naturals, for example, 33480 + ... + 34120.

Almost surely, 2^21665800 is an apocalyptic number.

21665800 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 21665800, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (27315495).

21665800 is an abundant number, since it is smaller than the sum of its proper divisors (32965190).

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

21665800 is a wasteful number, since it uses less digits than its factorization.

21665800 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 683 (or 661 counting only the distinct ones).

The product of its (nonzero) digits is 2880, while the sum is 28.

The square root of 21665800 is about 4654.6535853917. The cubic root of 21665800 is about 278.7778364420. Note that the first 3 decimals are identical.

The spelling of 21665800 in words is "twenty-one million, six hundred sixty-five thousand, eight hundred".