21105909 has 18 divisors (see below), whose sum is σ = 33511478. Its totient is φ = 12790800.

The previous prime is 21105907. The next prime is 21105911. The reversal of 21105909 is 90950112.

It is a happy number.

It is an interprime number because it is at equal distance from previous prime (21105907) and next prime (21105911).

It can be written as a sum of positive squares in only one way, i.e., 19847025 + 1258884 = 4455^2 + 1122^2 .

It is not a de Polignac number, because 21105909 - 2¹ = 21105907 is a prime.

It is a hoax number, since the sum of its digits (27) coincides with the sum of the digits of its distinct prime factors.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21105907) by changing a digit.

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 17 ways as a sum of consecutive naturals, for example, 8602 + ... + 10779.

Almost surely, 2^21105909 is an apocalyptic number.

It is an amenable number.

21105909 is a deficient number, since it is larger than the sum of its proper divisors (12405569).

21105909 is a wasteful number, since it uses less digits than its factorization.

21105909 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 19409 (or 19395 counting only the distinct ones).

The product of its (nonzero) digits is 810, while the sum is 27.

The square root of 21105909 is about 4594.1167812758. The cubic root of 21105909 is about 276.3554415505.

The spelling of 21105909 in words is "twenty-one million, one hundred five thousand, nine hundred nine".