21981564 has 48 divisors (see below), whose sum is σ = 62173440. Its totient is φ = 6660720.

The previous prime is 21981563. The next prime is 21981569. The reversal of 21981564 is 46518912.

21981564 is a `hidden beast` number, since 2 + 1 + 98 + 1 + 564 = 666.

It is a Harshad number since it is a multiple of its sum of digits (36).

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21981563) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 8064 + ... + 10439.

It is an arithmetic number, because the mean of its divisors is an integer number (1295280).

Almost surely, 2^21981564 is an apocalyptic number.

It is an amenable number.

21981564 is an abundant number, since it is smaller than the sum of its proper divisors (40191876).

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

21981564 is a wasteful number, since it uses less digits than its factorization.

21981564 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 18527 (or 18519 counting only the distinct ones).

The product of its digits is 17280, while the sum is 36.

The square root of 21981564 is about 4688.4500637204. The cubic root of 21981564 is about 280.1256408932.

The spelling of 21981564 in words is "twenty-one million, nine hundred eighty-one thousand, five hundred sixty-four".