21645020 has 48 divisors (see below), whose sum is σ = 47809440. Its totient is φ = 8219904.

The previous prime is 21644969. The next prime is 21645023. The reversal of 21645020 is 2054612.

It is a happy number.

It is a Harshad number since it is a multiple of its sum of digits (20).

It is a junction number, because it is equal to n+sod(n) for n = 21644983 and 21645001.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21645023) 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, 38582 + ... + 39138.

It is an arithmetic number, because the mean of its divisors is an integer number (996030).

Almost surely, 2^21645020 is an apocalyptic number.

21645020 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 21645020, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (23904720).

21645020 is an abundant number, since it is smaller than the sum of its proper divisors (26164420).

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

21645020 is a wasteful number, since it uses less digits than its factorization.

21645020 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 662 (or 660 counting only the distinct ones).

The product of its (nonzero) digits is 480, while the sum is 20.

The square root of 21645020 is about 4652.4208752004. The cubic root of 21645020 is about 278.6886812376.

Adding to 21645020 its reverse (2054612), we get a palindrome (23699632).

The spelling of 21645020 in words is "twenty-one million, six hundred forty-five thousand, twenty".