21205509 has 16 divisors (see below), whose sum is σ = 30532992. Its totient is φ = 13013568.

The previous prime is 21205453. The next prime is 21205523. The reversal of 21205509 is 90550212.

It is not a de Polignac number, because 21205509 - 2⁷ = 21205381 is a prime.

It is a super-2 number, since 2×21205509² = 899347223898162, which contains 22 as substring.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21205529) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 23464 + ... + 24350.

It is an arithmetic number, because the mean of its divisors is an integer number (1908312).

Almost surely, 2^21205509 is an apocalyptic number.

It is an amenable number.

21205509 is a deficient number, since it is larger than the sum of its proper divisors (9327483).

21205509 is a wasteful number, since it uses less digits than its factorization.

21205509 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1516.

The product of its (nonzero) digits is 900, while the sum is 24.

The square root of 21205509 is about 4604.9439736005. The cubic root of 21205509 is about 276.7894719411.

The spelling of 21205509 in words is "twenty-one million, two hundred five thousand, five hundred nine".