21066509 has 2 divisors, whose sum is σ = 21066510. Its totient is φ = 21066508.

The previous prime is 21066491. The next prime is 21066511. The reversal of 21066509 is 90566012.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 12096484 + 8970025 = 3478^2 + 2995^2 .

It is a cyclic number.

It is not a de Polignac number, because 21066509 - 2¹² = 21062413 is a prime.

Together with 21066511, 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 (21066109) 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, 10533254 + 10533255.

It is an arithmetic number, because the mean of its divisors is an integer number (10533255).

Almost surely, 2^21066509 is an apocalyptic number.

It is an amenable number.

21066509 is a deficient number, since it is larger than the sum of its proper divisors (1).

21066509 is an equidigital number, since it uses as much as digits as its factorization.

21066509 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 3240, while the sum is 29.

The square root of 21066509 is about 4589.8266851810. The cubic root of 21066509 is about 276.1833698835.

The spelling of 21066509 in words is "twenty-one million, sixty-six thousand, five hundred nine".