21009869 has 2 divisors, whose sum is σ = 21009870. Its totient is φ = 21009868.

The previous prime is 21009851. The next prime is 21009871. The reversal of 21009869 is 96890012.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 20811844 + 198025 = 4562^2 + 445^2 .

It is a cyclic number.

It is not a de Polignac number, because 21009869 - 2⁸ = 21009613 is a prime.

It is a super-2 number, since 2×21009869² = 882829190794322, which contains 22 as substring.

Together with 21009871, 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 (21003869) 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, 10504934 + 10504935.

It is an arithmetic number, because the mean of its divisors is an integer number (10504935).

Almost surely, 2^21009869 is an apocalyptic number.

It is an amenable number.

21009869 is a deficient number, since it is larger than the sum of its proper divisors (1).

21009869 is an equidigital number, since it uses as much as digits as its factorization.

21009869 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 7776, while the sum is 35.

The square root of 21009869 is about 4583.6523646542. The cubic root of 21009869 is about 275.9356296358.

The spelling of 21009869 in words is "twenty-one million, nine thousand, eight hundred sixty-nine".