21815986 has 4 divisors (see below), whose sum is σ = 32723982. Its totient is φ = 10907992.

The previous prime is 21815939. The next prime is 21815987. The reversal of 21815986 is 68951812.

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., 21206025 + 609961 = 4605^2 + 781^2 .

It is a Smith number, since the sum of its digits (40) 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 (21815987) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 5453995 + ... + 5453998.

Almost surely, 2^21815986 is an apocalyptic number.

21815986 is a deficient number, since it is larger than the sum of its proper divisors (10907996).

21815986 is a wasteful number, since it uses less digits than its factorization.

21815986 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 10907995.

The product of its digits is 34560, while the sum is 40.

The square root of 21815986 is about 4670.7586107612. The cubic root of 21815986 is about 279.4205106973.

It can be divided in two parts, 21815 and 986, that added together give a square (22801 = 151²).

The spelling of 21815986 in words is "twenty-one million, eight hundred fifteen thousand, nine hundred eighty-six".