21009820 has 48 divisors (see below), whose sum is σ = 50027040. Its totient is φ = 7347456.

The previous prime is 21009761. The next prime is 21009827. The reversal of 21009820 is 2890012.

It is a junction number, because it is equal to n+sod(n) for n = 21009791 and 21009800.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21009827) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 2814 + ... + 7066.

It is an arithmetic number, because the mean of its divisors is an integer number (1042230).

Almost surely, 2^21009820 is an apocalyptic number.

21009820 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 21009820, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (25013520).

21009820 is an abundant number, since it is smaller than the sum of its proper divisors (29017220).

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

21009820 is a wasteful number, since it uses less digits than its factorization.

21009820 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4294 (or 4292 counting only the distinct ones).

The product of its (nonzero) digits is 288, while the sum is 22.

The square root of 21009820 is about 4583.6470195686. The cubic root of 21009820 is about 275.9354151199.

Adding to 21009820 its reverse (2890012), we get a palindrome (23899832).

The spelling of 21009820 in words is "twenty-one million, nine thousand, eight hundred twenty".