21261720 has 64 divisors (see below), whose sum is σ = 64235520. Its totient is φ = 5629824.

The previous prime is 21261707. The next prime is 21261767. The reversal of 21261720 is 2716212.

It is a zygodrome in base 9 and base 11.

It is a congruent number.

It is an unprimeable number.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 19017 + ... + 20103.

It is an arithmetic number, because the mean of its divisors is an integer number (1003680).

Almost surely, 2^21261720 is an apocalyptic number.

21261720 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 21261720, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (32117760).

21261720 is an abundant number, since it is smaller than the sum of its proper divisors (42973800).

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

21261720 is a wasteful number, since it uses less digits than its factorization.

21261720 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1264 (or 1260 counting only the distinct ones).

The product of its (nonzero) digits is 336, while the sum is 21.

The square root of 21261720 is about 4611.0432658998. The cubic root of 21261720 is about 277.0338248867.

Adding to 21261720 its reverse (2716212), we get a palindrome (23977932).

The spelling of 21261720 in words is "twenty-one million, two hundred sixty-one thousand, seven hundred twenty".