21687778 has 16 divisors (see below), whose sum is σ = 39136320. Its totient is φ = 8805456.

The previous prime is 21687773. The next prime is 21687779. The reversal of 21687778 is 87778612.

It is not an unprimeable number, because it can be changed into a prime (21687773) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 40501 + ... + 41032.

It is an arithmetic number, because the mean of its divisors is an integer number (2446020).

Almost surely, 2^21687778 is an apocalyptic number.

21687778 is a deficient number, since it is larger than the sum of its proper divisors (17448542).

21687778 is a wasteful number, since it uses less digits than its factorization.

21687778 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 81561.

The product of its digits is 263424, while the sum is 46.

The square root of 21687778 is about 4657.0138500975. The cubic root of 21687778 is about 278.8720695760.

The spelling of 21687778 in words is "twenty-one million, six hundred eighty-seven thousand, seven hundred seventy-eight".