21918005 has 4 divisors (see below), whose sum is σ = 26301612. Its totient is φ = 17534400.

The previous prime is 21917999. The next prime is 21918007. The reversal of 21918005 is 50081912.

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 7187761 + 14730244 = 2681^2 + 3838^2 .

It is not a de Polignac number, because 21918005 - 2⁴ = 21917989 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21918007) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2191796 + ... + 2191805.

It is an arithmetic number, because the mean of its divisors is an integer number (6575403).

Almost surely, 2^21918005 is an apocalyptic number.

It is an amenable number.

21918005 is a deficient number, since it is larger than the sum of its proper divisors (4383607).

21918005 is an equidigital number, since it uses as much as digits as its factorization.

21918005 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4383606.

The product of its (nonzero) digits is 720, while the sum is 26.

The square root of 21918005 is about 4681.6669039990. Note that the first 3 decimals coincide. The cubic root of 21918005 is about 279.8553887611.

Adding to 21918005 its reverse (50081912), we get a palindrome (71999917).

The spelling of 21918005 in words is "twenty-one million, nine hundred eighteen thousand, five".