21087820 has 72 divisors (see below), whose sum is σ = 50912064. Its totient is φ = 7308288.

The previous prime is 21087809. The next prime is 21087827. The reversal of 21087820 is 2878012.

It is not an unprimeable number, because it can be changed into a prime (21087827) 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, 57277 + ... + 57643.

It is an arithmetic number, because the mean of its divisors is an integer number (707112).

Almost surely, 2^21087820 is an apocalyptic number.

21087820 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 21087820, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (25456032).

21087820 is an abundant number, since it is smaller than the sum of its proper divisors (29824244).

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

21087820 is a wasteful number, since it uses less digits than its factorization.

21087820 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 419 (or 404 counting only the distinct ones).

The product of its (nonzero) digits is 1792, while the sum is 28.

The square root of 21087820 is about 4592.1476457100. The cubic root of 21087820 is about 276.2764680522.

The spelling of 21087820 in words is "twenty-one million, eighty-seven thousand, eight hundred twenty".