21720981 has 8 divisors (see below), whose sum is σ = 28982944. Its totient is φ = 14469840.

The previous prime is 21720953. The next prime is 21720983. The reversal of 21720981 is 18902712.

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 21720981 - 2⁹ = 21720469 is a prime.

It is a super-2 number, since 2×21720981² = 943602031204722, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21720983) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 5826 + ... + 8796.

It is an arithmetic number, because the mean of its divisors is an integer number (3622868).

Almost surely, 2^21720981 is an apocalyptic number.

It is an amenable number.

21720981 is a deficient number, since it is larger than the sum of its proper divisors (7261963).

21720981 is a wasteful number, since it uses less digits than its factorization.

21720981 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 5411.

The product of its (nonzero) digits is 2016, while the sum is 30.

The square root of 21720981 is about 4660.5773247528. The cubic root of 21720981 is about 279.0143104901.

The spelling of 21720981 in words is "twenty-one million, seven hundred twenty thousand, nine hundred eighty-one".