21684720 has 40 divisors (see below), whose sum is σ = 67223376. Its totient is φ = 5782528.

The previous prime is 21684713. The next prime is 21684739. The reversal of 21684720 is 2748612.

21684720 = 118² + 119² + ... + 405².

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

It is a hoax number, since the sum of its digits (30) coincides with the sum of the digits of its distinct prime factors.

It is a Harshad number since it is a multiple of its sum of digits (30).

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 44937 + ... + 45416.

Almost surely, 2^21684720 is an apocalyptic number.

21684720 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

21684720 is an abundant number, since it is smaller than the sum of its proper divisors (45538656).

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

21684720 is a wasteful number, since it uses less digits than its factorization.

21684720 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 90369 (or 90363 counting only the distinct ones).

The product of its (nonzero) digits is 5376, while the sum is 30.

The square root of 21684720 is about 4656.6855165450. The cubic root of 21684720 is about 278.8589618727.

The spelling of 21684720 in words is "twenty-one million, six hundred eighty-four thousand, seven hundred twenty".