21090640 has 40 divisors (see below), whose sum is σ = 50184288. Its totient is φ = 8238720.

The previous prime is 21090631. The next prime is 21090647. The reversal of 21090640 is 4609012.

It is a tau number, because it is divible by the number of its divisors (40).

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21090647) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 375 + ... + 6505.

Almost surely, 2^21090640 is an apocalyptic number.

21090640 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 21090640, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (25092144).

21090640 is an abundant number, since it is smaller than the sum of its proper divisors (29093648).

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

21090640 is a wasteful number, since it uses less digits than its factorization.

21090640 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 6187 (or 6181 counting only the distinct ones).

The product of its (nonzero) digits is 432, while the sum is 22.

The square root of 21090640 is about 4592.4546813224. The cubic root of 21090640 is about 276.2887826634.

Adding to 21090640 its reverse (4609012), we get a palindrome (25699652).

The spelling of 21090640 in words is "twenty-one million, ninety thousand, six hundred forty".