21381209 has 2 divisors, whose sum is σ = 21381210. Its totient is φ = 21381208.

The previous prime is 21381203. The next prime is 21381211. The reversal of 21381209 is 90218312.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 21187609 + 193600 = 4603^2 + 440^2 .

It is a cyclic number.

It is not a de Polignac number, because 21381209 - 2²⁰ = 20332633 is a prime.

Together with 21381211, it forms a pair of twin primes.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (21381203) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10690604 + 10690605.

It is an arithmetic number, because the mean of its divisors is an integer number (10690605).

Almost surely, 2^21381209 is an apocalyptic number.

It is an amenable number.

21381209 is a deficient number, since it is larger than the sum of its proper divisors (1).

21381209 is an equidigital number, since it uses as much as digits as its factorization.

21381209 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 864, while the sum is 26.

The square root of 21381209 is about 4623.9819420063. The cubic root of 21381209 is about 277.5518243171.

The spelling of 21381209 in words is "twenty-one million, three hundred eighty-one thousand, two hundred nine".