21309990 has 64 divisors (see below), whose sum is σ = 55726272. Its totient is φ = 5184000.

The previous prime is 21309973. The next prime is 21309991. The reversal of 21309990 is 9990312.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21309991) by changing a digit.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 39120 + ... + 39660.

It is an arithmetic number, because the mean of its divisors is an integer number (870723).

Almost surely, 2^21309990 is an apocalyptic number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21309990, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (27863136).

21309990 is an abundant number, since it is smaller than the sum of its proper divisors (34416282).

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

21309990 is a wasteful number, since it uses less digits than its factorization.

21309990 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 665.

The product of its (nonzero) digits is 4374, while the sum is 33.

The square root of 21309990 is about 4616.2744719091. The cubic root of 21309990 is about 277.2433143004.

The spelling of 21309990 in words is "twenty-one million, three hundred nine thousand, nine hundred ninety".