21387990 has 64 divisors (see below), whose sum is σ = 55774656. Its totient is φ = 5217792.

The previous prime is 21387973. The next prime is 21388009. The reversal of 21387990 is 9978312.

It is a super-2 number, since 2×21387990² = 914892232480200, which contains 22 as substring.

It is a Harshad number since it is a multiple of its sum of digits (39).

It is a congruent number.

It is an unprimeable number.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 67312 + ... + 67628.

It is an arithmetic number, because the mean of its divisors is an integer number (871479).

Almost surely, 2^21387990 is an apocalyptic number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21387990, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (27887328).

21387990 is an abundant number, since it is smaller than the sum of its proper divisors (34386666).

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

21387990 is a wasteful number, since it uses less digits than its factorization.

21387990 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 513.

The product of its (nonzero) digits is 27216, while the sum is 39.

The square root of 21387990 is about 4624.7151263618. The cubic root of 21387990 is about 277.5811628513.

The spelling of 21387990 in words is "twenty-one million, three hundred eighty-seven thousand, nine hundred ninety".