21431209 has 2 divisors, whose sum is σ = 21431210. Its totient is φ = 21431208.

The previous prime is 21431197. The next prime is 21431231. The reversal of 21431209 is 90213412.

It is an a-pointer prime, because the next prime (21431231) can be obtained adding 21431209 to its sum of digits (22).

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 12110400 + 9320809 = 3480^2 + 3053^2 .

It is a cyclic number.

It is not a de Polignac number, because 21431209 - 2⁹ = 21430697 is a prime.

It is a Chen prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21431209.

It is not a weakly prime, because it can be changed into another prime (21431269) 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, 10715604 + 10715605.

It is an arithmetic number, because the mean of its divisors is an integer number (10715605).

Almost surely, 2^21431209 is an apocalyptic number.

It is an amenable number.

21431209 is a deficient number, since it is larger than the sum of its proper divisors (1).

21431209 is an equidigital number, since it uses as much as digits as its factorization.

21431209 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 432, while the sum is 22.

The square root of 21431209 is about 4629.3853803718. The cubic root of 21431209 is about 277.7680077216.

The spelling of 21431209 in words is "twenty-one million, four hundred thirty-one thousand, two hundred nine".