21405905 has 8 divisors (see below), whose sum is σ = 26172072. Its totient is φ = 16801408.

The previous prime is 21405899. The next prime is 21405931. The reversal of 21405905 is 50950412.

It can be written as a sum of positive squares in 4 ways, for example, as 10804369 + 10601536 = 3287^2 + 3256^2 .

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 21405905 - 2⁴ = 21405889 is a prime.

It is a Duffinian number.

It is an unprimeable number.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 40124 + ... + 40653.

It is an arithmetic number, because the mean of its divisors is an integer number (3271509).

Almost surely, 2^21405905 is an apocalyptic number.

It is an amenable number.

21405905 is a deficient number, since it is larger than the sum of its proper divisors (4766167).

21405905 is an equidigital number, since it uses as much as digits as its factorization.

21405905 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 80835.

The product of its (nonzero) digits is 1800, while the sum is 26.

The square root of 21405905 is about 4626.6515969976. The cubic root of 21405905 is about 277.6586436979.

The spelling of 21405905 in words is "twenty-one million, four hundred five thousand, nine hundred five".