21978165 has 16 divisors (see below), whose sum is σ = 38362176. Its totient is φ = 10656000.

The previous prime is 21978139. The next prime is 21978167. The reversal of 21978165 is 56187912.

21978165 = T₁₄ + T₁₅ + ... + T₅₀₈.

It is not a de Polignac number, because 21978165 - 2¹⁴ = 21961781 is a prime.

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21978167) 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, 66436 + ... + 66765.

It is an arithmetic number, because the mean of its divisors is an integer number (2397636).

Almost surely, 2^21978165 is an apocalyptic number.

It is an amenable number.

21978165 is a deficient number, since it is larger than the sum of its proper divisors (16384011).

21978165 is a wasteful number, since it uses less digits than its factorization.

21978165 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 133220.

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

The square root of 21978165 is about 4688.0875631754. The cubic root of 21978165 is about 280.1112015789. Note that the first 3 decimals are identical.

The spelling of 21978165 in words is "twenty-one million, nine hundred seventy-eight thousand, one hundred sixty-five".