21721830 has 32 divisors (see below), whose sum is σ = 56143584. Its totient is φ = 5346816.

The previous prime is 21721823. The next prime is 21721831. The reversal of 21721830 is 3812712.

It is a zygodrome in base 9.

It is a self number, because there is not a number n which added to its sum of digits gives 21721830.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21721831) 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, 27459 + ... + 28238.

It is an arithmetic number, because the mean of its divisors is an integer number (1754487).

Almost surely, 2^21721830 is an apocalyptic number.

21721830 is an abundant number, since it is smaller than the sum of its proper divisors (34421754).

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

21721830 is a wasteful number, since it uses less digits than its factorization.

21721830 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 55720.

The product of its (nonzero) digits is 672, while the sum is 24.

The square root of 21721830 is about 4660.6684069992. The cubic root of 21721830 is about 279.0179456860.

The spelling of 21721830 in words is "twenty-one million, seven hundred twenty-one thousand, eight hundred thirty".