21205828 has 18 divisors (see below), whose sum is σ = 43169406. Its totient is φ = 9088128.

The previous prime is 21205823. The next prime is 21205853. The reversal of 21205828 is 82850212.

It can be written as a sum of positive squares in only one way, i.e., 17994564 + 3211264 = 4242^2 + 1792^2 .

It is a Harshad number since it is a multiple of its sum of digits (28).

It is a junction number, because it is equal to n+sod(n) for n = 21205796 and 21205805.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21205823) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 53901 + ... + 54292.

Almost surely, 2^21205828 is an apocalyptic number.

21205828 is a gapful number since it is divisible by the number (28) formed by its first and last digit.

It is an amenable number.

21205828 is an abundant number, since it is smaller than the sum of its proper divisors (21963578).

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

21205828 is a wasteful number, since it uses less digits than its factorization.

21205828 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 108211 (or 108202 counting only the distinct ones).

The product of its (nonzero) digits is 2560, while the sum is 28.

The square root of 21205828 is about 4604.9786101566. The cubic root of 21205828 is about 276.7908598728.

The spelling of 21205828 in words is "twenty-one million, two hundred five thousand, eight hundred twenty-eight".