21705640 has 32 divisors (see below), whose sum is σ = 53278560. Its totient is φ = 7892800.

The previous prime is 21705631. The next prime is 21705641. The reversal of 21705640 is 4650712.

It is a super-2 number, since 2×21705640² = 942269615619200, which contains 22 as substring.

It is not an unprimeable number, because it can be changed into a prime (21705641) 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, 24226 + ... + 25105.

It is an arithmetic number, because the mean of its divisors is an integer number (1664955).

Almost surely, 2^21705640 is an apocalyptic number.

21705640 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

21705640 is an abundant number, since it is smaller than the sum of its proper divisors (31572920).

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

21705640 is a wasteful number, since it uses less digits than its factorization.

21705640 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 49353 (or 49349 counting only the distinct ones).

The product of its (nonzero) digits is 1680, while the sum is 25.

The square root of 21705640 is about 4658.9312079060. The cubic root of 21705640 is about 278.9486080151.

The spelling of 21705640 in words is "twenty-one million, seven hundred five thousand, six hundred forty".