21861209 has 2 divisors, whose sum is σ = 21861210. Its totient is φ = 21861208.

The previous prime is 21861199. The next prime is 21861221. The reversal of 21861209 is 90216812.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 14462809 + 7398400 = 3803^2 + 2720^2 .

It is a cyclic number.

It is not a de Polignac number, because 21861209 - 2⁴ = 21861193 is a prime.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (21861239) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10930604 + 10930605.

It is an arithmetic number, because the mean of its divisors is an integer number (10930605).

Almost surely, 2^21861209 is an apocalyptic number.

It is an amenable number.

21861209 is a deficient number, since it is larger than the sum of its proper divisors (1).

21861209 is an equidigital number, since it uses as much as digits as its factorization.

21861209 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 1728, while the sum is 29.

The square root of 21861209 is about 4675.5971811096. The cubic root of 21861209 is about 279.6134504443.

The spelling of 21861209 in words is "twenty-one million, eight hundred sixty-one thousand, two hundred nine".