21987578 has 4 divisors (see below), whose sum is σ = 32981370. Its totient is φ = 10993788.

The previous prime is 21987571. The next prime is 21987601. The reversal of 21987578 is 87578912.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in only one way, i.e., 21501769 + 485809 = 4637^2 + 697^2 .

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (21987571) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 5496893 + ... + 5496896.

Almost surely, 2^21987578 is an apocalyptic number.

21987578 is a deficient number, since it is larger than the sum of its proper divisors (10993792).

21987578 is a wasteful number, since it uses less digits than its factorization.

21987578 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 10993791.

The product of its digits is 282240, while the sum is 47.

The square root of 21987578 is about 4689.0913831999. The cubic root of 21987578 is about 280.1511853599.

The spelling of 21987578 in words is "twenty-one million, nine hundred eighty-seven thousand, five hundred seventy-eight".