21664609 has 2 divisors, whose sum is σ = 21664610. Its totient is φ = 21664608.

The previous prime is 21664607. The next prime is 21664637. The reversal of 21664609 is 90646612.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 13712209 + 7952400 = 3703^2 + 2820^2 .

It is a cyclic number.

It is not a de Polignac number, because 21664609 - 2¹ = 21664607 is a prime.

Together with 21664607, it forms a pair of twin primes.

It is not a weakly prime, because it can be changed into another prime (21664607) 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, 10832304 + 10832305.

It is an arithmetic number, because the mean of its divisors is an integer number (10832305).

Almost surely, 2^21664609 is an apocalyptic number.

It is an amenable number.

21664609 is a deficient number, since it is larger than the sum of its proper divisors (1).

21664609 is an equidigital number, since it uses as much as digits as its factorization.

21664609 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 15552, while the sum is 34.

The square root of 21664609 is about 4654.5256471525. The cubic root of 21664609 is about 278.7727280764.

The spelling of 21664609 in words is "twenty-one million, six hundred sixty-four thousand, six hundred nine".