21978561 has 24 divisors (see below), whose sum is σ = 32481792. Its totient is φ = 13208800.

The previous prime is 21978553. The next prime is 21978571. The reversal of 21978561 is 16587912.

It is not a de Polignac number, because 21978561 - 2³ = 21978553 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21978521) by changing a digit.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 69175 + ... + 69491.

It is an arithmetic number, because the mean of its divisors is an integer number (1353408).

Almost surely, 2^21978561 is an apocalyptic number.

It is an amenable number.

21978561 is a deficient number, since it is larger than the sum of its proper divisors (10503231).

21978561 is a wasteful number, since it uses less digits than its factorization.

21978561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 533 (or 522 counting only the distinct ones).

The product of its digits is 30240, while the sum is 39.

The square root of 21978561 is about 4688.1297976912. The cubic root of 21978561 is about 280.1128839057.

The spelling of 21978561 in words is "twenty-one million, nine hundred seventy-eight thousand, five hundred sixty-one".