21185905 has 16 divisors (see below), whose sum is σ = 27481104. Its totient is φ = 15586560.

The previous prime is 21185891. The next prime is 21185917. The reversal of 21185905 is 50958112.

It can be written as a sum of positive squares in 8 ways, for example, as 1817104 + 19368801 = 1348^2 + 4401^2 .

It is not a de Polignac number, because 21185905 - 2⁵ = 21185873 is a prime.

It is a Duffinian number.

It is an unprimeable number.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 25395 + ... + 26215.

It is an arithmetic number, because the mean of its divisors is an integer number (1717569).

Almost surely, 2^21185905 is an apocalyptic number.

It is an amenable number.

21185905 is a deficient number, since it is larger than the sum of its proper divisors (6295199).

21185905 is a wasteful number, since it uses less digits than its factorization.

21185905 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1236.

The product of its (nonzero) digits is 3600, while the sum is 31.

The square root of 21185905 is about 4602.8148996022. The cubic root of 21185905 is about 276.7041504995.

The spelling of 21185905 in words is "twenty-one million, one hundred eighty-five thousand, nine hundred five".