21809197 has 2 divisors, whose sum is σ = 21809198. Its totient is φ = 21809196.

The previous prime is 21809191. The next prime is 21809219. The reversal of 21809197 is 79190812.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 21473956 + 335241 = 4634^2 + 579^2 .

It is a cyclic number.

It is not a de Polignac number, because 21809197 - 2⁷ = 21809069 is a prime.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21809191) 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, 10904598 + 10904599.

It is an arithmetic number, because the mean of its divisors is an integer number (10904599).

Almost surely, 2^21809197 is an apocalyptic number.

It is an amenable number.

21809197 is a deficient number, since it is larger than the sum of its proper divisors (1).

21809197 is an equidigital number, since it uses as much as digits as its factorization.

21809197 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 9072, while the sum is 37.

The square root of 21809197 is about 4670.0317986069. The cubic root of 21809197 is about 279.3915230449.

The spelling of 21809197 in words is "twenty-one million, eight hundred nine thousand, one hundred ninety-seven".