21978515 has 16 divisors (see below), whose sum is σ = 28576800. Its totient is φ = 16131072.

The previous prime is 21978487. The next prime is 21978521. The reversal of 21978515 is 51587912.

It is a cyclic number.

It is not a de Polignac number, because 21978515 - 2¹⁰ = 21977491 is a prime.

It is an unprimeable number.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 10691 + ... + 12579.

It is an arithmetic number, because the mean of its divisors is an integer number (1786050).

Almost surely, 2^21978515 is an apocalyptic number.

21978515 is a deficient number, since it is larger than the sum of its proper divisors (6598285).

21978515 is a wasteful number, since it uses less digits than its factorization.

21978515 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2086.

The product of its digits is 25200, while the sum is 38.

The square root of 21978515 is about 4688.1248916811. The cubic root of 21978515 is about 280.1126884850.

The spelling of 21978515 in words is "twenty-one million, nine hundred seventy-eight thousand, five hundred fifteen".