21212820 has 96 divisors (see below), whose sum is σ = 66675840. Its totient is φ = 5598720.

The previous prime is 21212809. The next prime is 21212837. The reversal of 21212820 is 2821212.

It is a happy number.

It is a Harshad number since it is a multiple of its sum of digits (18).

It is a junction number, because it is equal to n+sod(n) for n = 21212793 and 21212802.

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, 87900 + ... + 88140.

It is an arithmetic number, because the mean of its divisors is an integer number (694540).

Almost surely, 2^21212820 is an apocalyptic number.

21212820 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 21212820, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (33337920).

21212820 is an abundant number, since it is smaller than the sum of its proper divisors (45463020).

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

21212820 is a wasteful number, since it uses less digits than its factorization.

21212820 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 422 (or 414 counting only the distinct ones).

The product of its (nonzero) digits is 128, while the sum is 18.

The square root of 21212820 is about 4605.7377259240. The cubic root of 21212820 is about 276.8212777533.

The spelling of 21212820 in words is "twenty-one million, two hundred twelve thousand, eight hundred twenty".