21530909 has 2 divisors, whose sum is σ = 21530910. Its totient is φ = 21530908.

The previous prime is 21530903. The next prime is 21530911. The reversal of 21530909 is 90903512.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 17825284 + 3705625 = 4222^2 + 1925^2 .

It is a cyclic number.

It is not a de Polignac number, because 21530909 - 2²⁰ = 20482333 is a prime.

Together with 21530911, it forms a pair of twin primes.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21530903) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10765454 + 10765455.

It is an arithmetic number, because the mean of its divisors is an integer number (10765455).

Almost surely, 2^21530909 is an apocalyptic number.

It is an amenable number.

21530909 is a deficient number, since it is larger than the sum of its proper divisors (1).

21530909 is an equidigital number, since it uses as much as digits as its factorization.

21530909 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 2430, while the sum is 29.

The square root of 21530909 is about 4640.1410538905. The cubic root of 21530909 is about 278.1980758029.

The spelling of 21530909 in words is "twenty-one million, five hundred thirty thousand, nine hundred nine".