21978287 has 8 divisors (see below), whose sum is σ = 22818432. Its totient is φ = 21146400.

The previous prime is 21978251. The next prime is 21978293. The reversal of 21978287 is 78287912.

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

It is a cyclic number.

It is not a de Polignac number, because 21978287 - 2¹⁸ = 21716143 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21978217) 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, 3653 + ... + 7569.

It is an arithmetic number, because the mean of its divisors is an integer number (2852304).

Almost surely, 2^21978287 is an apocalyptic number.

21978287 is a deficient number, since it is larger than the sum of its proper divisors (840145).

21978287 is a wasteful number, since it uses less digits than its factorization.

21978287 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4129.

The product of its digits is 112896, while the sum is 44.

The square root of 21978287 is about 4688.1005748597. The cubic root of 21978287 is about 280.1117198737. Note that the first 3 decimals are identical.

The spelling of 21978287 in words is "twenty-one million, nine hundred seventy-eight thousand, two hundred eighty-seven".