21697840 has 40 divisors (see below), whose sum is σ = 51313680. Its totient is φ = 8530176.

The previous prime is 21697811. The next prime is 21697847. The reversal of 21697840 is 4879612.

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

It is a self number, because there is not a number n which added to its sum of digits gives 21697840.

It is not an unprimeable number, because it can be changed into a prime (21697847) 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, 2422 + ... + 7018.

It is an arithmetic number, because the mean of its divisors is an integer number (1282842).

Almost surely, 2^21697840 is an apocalyptic number.

21697840 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 21697840, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (25656840).

21697840 is an abundant number, since it is smaller than the sum of its proper divisors (29615840).

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

21697840 is a wasteful number, since it uses less digits than its factorization.

21697840 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4669 (or 4663 counting only the distinct ones).

The product of its (nonzero) digits is 24192, while the sum is 37.

The square root of 21697840 is about 4658.0940308242. The cubic root of 21697840 is about 278.9151902825.

The spelling of 21697840 in words is "twenty-one million, six hundred ninety-seven thousand, eight hundred forty".