21818381 has 4 divisors (see below), whose sum is σ = 23496732. Its totient is φ = 20140032.

The previous prime is 21818359. The next prime is 21818393. The reversal of 21818381 is 18381812.

It is a happy number.

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 9357481 + 12460900 = 3059^2 + 3530^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21818381 is a prime.

It is a super-2 number, since 2×21818381² = 952083498922322, which contains 22 as substring.

It is an alternating number because its digits alternate between even and odd.

It is a Duffinian number.

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21818311) 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, 839156 + ... + 839181.

It is an arithmetic number, because the mean of its divisors is an integer number (5874183).

Almost surely, 2^21818381 is an apocalyptic number.

It is an amenable number.

21818381 is a deficient number, since it is larger than the sum of its proper divisors (1678351).

21818381 is a wasteful number, since it uses less digits than its factorization.

21818381 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1678350.

The product of its digits is 3072, while the sum is 32.

The square root of 21818381 is about 4671.0149860603. The cubic root of 21818381 is about 279.4307354263.

The spelling of 21818381 in words is "twenty-one million, eight hundred eighteen thousand, three hundred eighty-one".