21209618 has 4 divisors (see below), whose sum is σ = 31814430. Its totient is φ = 10604808.

The previous prime is 21209609. The next prime is 21209623. The reversal of 21209618 is 81690212.

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., 17197609 + 4012009 = 4147^2 + 2003^2 .

It is an unprimeable number.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 5302403 + ... + 5302406.

Almost surely, 2^21209618 is an apocalyptic number.

21209618 is a deficient number, since it is larger than the sum of its proper divisors (10604812).

21209618 is a wasteful number, since it uses less digits than its factorization.

21209618 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 10604811.

The product of its (nonzero) digits is 1728, while the sum is 29.

The square root of 21209618 is about 4605.3901029120. The cubic root of 21209618 is about 276.8073486555.

The spelling of 21209618 in words is "twenty-one million, two hundred nine thousand, six hundred eighteen".