21206609 has 2 divisors, whose sum is σ = 21206610. Its totient is φ = 21206608.

The previous prime is 21206593. The next prime is 21206617. The reversal of 21206609 is 90660212.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 17875984 + 3330625 = 4228^2 + 1825^2 .

It is a cyclic number.

It is not a de Polignac number, because 21206609 - 2⁴ = 21206593 is a prime.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (21206699) 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, 10603304 + 10603305.

It is an arithmetic number, because the mean of its divisors is an integer number (10603305).

Almost surely, 2^21206609 is an apocalyptic number.

It is an amenable number.

21206609 is a deficient number, since it is larger than the sum of its proper divisors (1).

21206609 is an equidigital number, since it uses as much as digits as its factorization.

21206609 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 1296, while the sum is 26.

The square root of 21206609 is about 4605.0634089011. The cubic root of 21206609 is about 276.7942578539.

The spelling of 21206609 in words is "twenty-one million, two hundred six thousand, six hundred nine".