21505009 has 2 divisors, whose sum is σ = 21505010. Its totient is φ = 21505008.

The previous prime is 21505007. The next prime is 21505013. The reversal of 21505009 is 90050512.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 17875984 + 3629025 = 4228^2 + 1905^2 .

It is a cyclic number.

It is not a de Polignac number, because 21505009 - 2¹ = 21505007 is a prime.

Together with 21505007, it forms a pair of twin primes.

It is not a weakly prime, because it can be changed into another prime (21505007) 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 as a sum of consecutive naturals, namely, 10752504 + 10752505.

It is an arithmetic number, because the mean of its divisors is an integer number (10752505).

Almost surely, 2^21505009 is an apocalyptic number.

It is an amenable number.

21505009 is a deficient number, since it is larger than the sum of its proper divisors (1).

21505009 is an equidigital number, since it uses as much as digits as its factorization.

21505009 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 450, while the sum is 22.

The square root of 21505009 is about 4637.3493506528. The cubic root of 21505009 is about 278.0864808667.

The spelling of 21505009 in words is "twenty-one million, five hundred five thousand, nine".