21251720 has 64 divisors (see below), whose sum is σ = 55468800. Its totient is φ = 7176960.

The previous prime is 21251719. The next prime is 21251731. The reversal of 21251720 is 2715212.

It is a Harshad number since it is a multiple of its sum of digits (20).

It is a plaindrome in base 16.

It is a junction number, because it is equal to n+sod(n) for n = 21251692 and 21251701.

It is an unprimeable number.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 19346 + ... + 20414.

It is an arithmetic number, because the mean of its divisors is an integer number (866700).

Almost surely, 2^21251720 is an apocalyptic number.

21251720 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 21251720, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (27734400).

21251720 is an abundant number, since it is smaller than the sum of its proper divisors (34217080).

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

21251720 is a wasteful number, since it uses less digits than its factorization.

21251720 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1158 (or 1154 counting only the distinct ones).

The product of its (nonzero) digits is 280, while the sum is 20.

The square root of 21251720 is about 4609.9587850652. The cubic root of 21251720 is about 276.9903857437.

Adding to 21251720 its reverse (2715212), we get a palindrome (23966932).

The spelling of 21251720 in words is "twenty-one million, two hundred fifty-one thousand, seven hundred twenty".