21172720 has 20 divisors (see below), whose sum is σ = 49226760. Its totient is φ = 8469056.

The previous prime is 21172717. The next prime is 21172733. The reversal of 21172720 is 2727112.

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

It is a junction number, because it is equal to n+sod(n) for n = 21172691 and 21172700.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 132250 + ... + 132409.

It is an arithmetic number, because the mean of its divisors is an integer number (2461338).

Almost surely, 2^21172720 is an apocalyptic number.

21172720 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

21172720 is an abundant number, since it is smaller than the sum of its proper divisors (28054040).

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

21172720 is a wasteful number, since it uses less digits than its factorization.

21172720 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 264672 (or 264666 counting only the distinct ones).

The product of its (nonzero) digits is 392, while the sum is 22.

The square root of 21172720 is about 4601.3824009747. The cubic root of 21172720 is about 276.6467365202.

Adding to 21172720 its reverse (2727112), we get a palindrome (23899832).

The spelling of 21172720 in words is "twenty-one million, one hundred seventy-two thousand, seven hundred twenty".