21518821 has 4 divisors (see below), whose sum is σ = 22784652. Its totient is φ = 20252992.

The previous prime is 21518813. The next prime is 21518837. The reversal of 21518821 is 12881512.

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

It can be written as a sum of positive squares in 2 ways, for example, as 10329796 + 11189025 = 3214^2 + 3345^2 .

It is a cyclic number.

It is not a de Polignac number, because 21518821 - 2³ = 21518813 is a prime.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 21518821.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21518881) 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, 632890 + ... + 632923.

It is an arithmetic number, because the mean of its divisors is an integer number (5696163).

Almost surely, 2^21518821 is an apocalyptic number.

It is an amenable number.

21518821 is a deficient number, since it is larger than the sum of its proper divisors (1265831).

21518821 is a wasteful number, since it uses less digits than its factorization.

21518821 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1265830.

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

The square root of 21518821 is about 4638.8383244084. The cubic root of 21518821 is about 278.1460035647.

The spelling of 21518821 in words is "twenty-one million, five hundred eighteen thousand, eight hundred twenty-one".