21209171 has 2 divisors, whose sum is σ = 21209172. Its totient is φ = 21209170.

The previous prime is 21209161. The next prime is 21209189. The reversal of 21209171 is 17190212.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 21209171 - 2¹⁸ = 20947027 is a prime.

It is not a weakly prime, because it can be changed into another prime (21209131) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10604585 + 10604586.

It is an arithmetic number, because the mean of its divisors is an integer number (10604586).

Almost surely, 2^21209171 is an apocalyptic number.

21209171 is a deficient number, since it is larger than the sum of its proper divisors (1).

21209171 is an equidigital number, since it uses as much as digits as its factorization.

21209171 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 252, while the sum is 23.

The square root of 21209171 is about 4605.3415725655. The cubic root of 21209171 is about 276.8054040385.

Adding to 21209171 its reverse (17190212), we get a palindrome (38399383).

The spelling of 21209171 in words is "twenty-one million, two hundred nine thousand, one hundred seventy-one".