21851987 has 4 divisors (see below), whose sum is σ = 23137416. Its totient is φ = 20566560.

The previous prime is 21851981. The next prime is 21851989. The reversal of 21851987 is 78915812.

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

It is a cyclic number.

It is not a de Polignac number, because 21851987 - 2⁶ = 21851923 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21851981) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 642689 + ... + 642722.

It is an arithmetic number, because the mean of its divisors is an integer number (5784354).

Almost surely, 2^21851987 is an apocalyptic number.

21851987 is a deficient number, since it is larger than the sum of its proper divisors (1285429).

21851987 is a wasteful number, since it uses less digits than its factorization.

21851987 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1285428.

The product of its digits is 40320, while the sum is 41.

The square root of 21851987 is about 4674.6108928979. The cubic root of 21851987 is about 279.5741272468.

The spelling of 21851987 in words is "twenty-one million, eight hundred fifty-one thousand, nine hundred eighty-seven".