21978405 has 32 divisors (see below), whose sum is σ = 39640320. Its totient is φ = 11551680.

The previous prime is 21978403. The next prime is 21978487. The reversal of 21978405 is 50487912.

It is not a de Polignac number, because 21978405 - 2¹ = 21978403 is a prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21978405.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21978401) 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, 8439 + ... + 10731.

It is an arithmetic number, because the mean of its divisors is an integer number (1238760).

Almost surely, 2^21978405 is an apocalyptic number.

It is an amenable number.

21978405 is a deficient number, since it is larger than the sum of its proper divisors (17661915).

21978405 is a wasteful number, since it uses less digits than its factorization.

21978405 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2378 (or 2372 counting only the distinct ones).

The product of its (nonzero) digits is 20160, while the sum is 36.

The square root of 21978405 is about 4688.1131598971. The cubic root of 21978405 is about 280.1122211733.

The spelling of 21978405 in words is "twenty-one million, nine hundred seventy-eight thousand, four hundred five".