21987878 has 24 divisors (see below), whose sum is σ = 37113384. Its totient is φ = 9757440.

The previous prime is 21987871. The next prime is 21987899. The reversal of 21987878 is 87878912.

It is a happy number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21987871) by changing a digit.

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 11 ways as a sum of consecutive naturals, for example, 9350 + ... + 11462.

It is an arithmetic number, because the mean of its divisors is an integer number (1546391).

Almost surely, 2^21987878 is an apocalyptic number.

21987878 is a deficient number, since it is larger than the sum of its proper divisors (15125506).

21987878 is a wasteful number, since it uses less digits than its factorization.

21987878 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2180 (or 2169 counting only the distinct ones).

The product of its digits is 451584, while the sum is 50.

The square root of 21987878 is about 4689.1233722307. The cubic root of 21987878 is about 280.1524594880.

The spelling of 21987878 in words is "twenty-one million, nine hundred eighty-seven thousand, eight hundred seventy-eight".