21681680 has 20 divisors (see below), whose sum is σ = 50410092. Its totient is φ = 8672640.

The previous prime is 21681659. The next prime is 21681683. The reversal of 21681680 is 8618612.

It can be written as a sum of positive squares in 2 ways, for example, as 5456896 + 16224784 = 2336^2 + 4028^2 .

It is a tau number, because it is divible by the number of its divisors (20).

It is not an unprimeable number, because it can be changed into a prime (21681683) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 135431 + ... + 135590.

Almost surely, 2^21681680 is an apocalyptic number.

21681680 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

21681680 is an abundant number, since it is smaller than the sum of its proper divisors (28728412).

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

21681680 is a wasteful number, since it uses less digits than its factorization.

21681680 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 271034 (or 271028 counting only the distinct ones).

The product of its (nonzero) digits is 4608, while the sum is 32.

The square root of 21681680 is about 4656.3590926817. The cubic root of 21681680 is about 278.8459301026.

The spelling of 21681680 in words is "twenty-one million, six hundred eighty-one thousand, six hundred eighty".