21521818 has 4 divisors (see below), whose sum is σ = 32282730. Its totient is φ = 10760908.

The previous prime is 21521803. The next prime is 21521821. The reversal of 21521818 is 81812512.

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., 11580409 + 9941409 = 3403^2 + 3153^2 .

It is an unprimeable number.

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 as a sum of consecutive naturals, namely, 5380453 + ... + 5380456.

Almost surely, 2^21521818 is an apocalyptic number.

21521818 is a deficient number, since it is larger than the sum of its proper divisors (10760912).

21521818 is a wasteful number, since it uses less digits than its factorization.

21521818 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 10760911.

The product of its digits is 1280, while the sum is 28.

The square root of 21521818 is about 4639.1613466229. The cubic root of 21521818 is about 278.1589157479.

The spelling of 21521818 in words is "twenty-one million, five hundred twenty-one thousand, eight hundred eighteen".