21961720 has 128 divisors (see below), whose sum is σ = 59097600. Its totient is φ = 7257600.

The previous prime is 21961697. The next prime is 21961721. The reversal of 21961720 is 2716912.

It is a happy number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21961721) by changing a digit.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 309285 + ... + 309355.

It is an arithmetic number, because the mean of its divisors is an integer number (461700).

Almost surely, 2^21961720 is an apocalyptic number.

21961720 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 21961720, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (29548800).

21961720 is an abundant number, since it is smaller than the sum of its proper divisors (37135880).

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

21961720 is a wasteful number, since it uses less digits than its factorization.

21961720 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 149 (or 145 counting only the distinct ones).

The product of its (nonzero) digits is 1512, while the sum is 28.

The square root of 21961720 is about 4686.3333214785. Note that the first 4 decimals coincide. The cubic root of 21961720 is about 280.0413204325.

It can be divided in two parts, 2196 and 1720, that added together give a triangular number (3916 = T₈₈).

The spelling of 21961720 in words is "twenty-one million, nine hundred sixty-one thousand, seven hundred twenty".