21987555 has 8 divisors (see below), whose sum is σ = 35180112. Its totient is φ = 11726688.

The previous prime is 21987547. The next prime is 21987557. The reversal of 21987555 is 55578912.

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

It is not a de Polignac number, because 21987555 - 2³ = 21987547 is a prime.

It is a Smith number, since the sum of its digits (42) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is not an unprimeable number, because it can be changed into a prime (21987557) 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, 732904 + ... + 732933.

It is an arithmetic number, because the mean of its divisors is an integer number (4397514).

Almost surely, 2^21987555 is an apocalyptic number.

21987555 is a deficient number, since it is larger than the sum of its proper divisors (13192557).

21987555 is a wasteful number, since it uses less digits than its factorization.

21987555 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1465845.

The product of its digits is 126000, while the sum is 42.

The square root of 21987555 is about 4689.0889306986. The cubic root of 21987555 is about 280.1510876763.

The spelling of 21987555 in words is "twenty-one million, nine hundred eighty-seven thousand, five hundred fifty-five".