21391778 has 8 divisors (see below), whose sum is σ = 32494080. Its totient is φ = 10560420.

The previous prime is 21391757. The next prime is 21391787. The reversal of 21391778 is 87719312.

It is a sphenic number, since it is the product of 3 distinct primes.

It is a Curzon number.

It is an unprimeable number.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 67538 + ... + 67853.

It is an arithmetic number, because the mean of its divisors is an integer number (4061760).

Almost surely, 2^21391778 is an apocalyptic number.

21391778 is a deficient number, since it is larger than the sum of its proper divisors (11102302).

21391778 is a wasteful number, since it uses less digits than its factorization.

21391778 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 135472.

The product of its digits is 21168, while the sum is 38.

The square root of 21391778 is about 4625.1246469690. The cubic root of 21391778 is about 277.5975492341.

The spelling of 21391778 in words is "twenty-one million, three hundred ninety-one thousand, seven hundred seventy-eight".