21666191 has 2 divisors, whose sum is σ = 21666192. Its totient is φ = 21666190.

The previous prime is 21666167. The next prime is 21666209. The reversal of 21666191 is 19166612.

It is a strong prime.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21666191 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 (21666101) 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, 10833095 + 10833096.

It is an arithmetic number, because the mean of its divisors is an integer number (10833096).

Almost surely, 2^21666191 is an apocalyptic number.

21666191 is a deficient number, since it is larger than the sum of its proper divisors (1).

21666191 is an equidigital number, since it uses as much as digits as its factorization.

21666191 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 3888, while the sum is 32.

The square root of 21666191 is about 4654.6955861796. The cubic root of 21666191 is about 278.7795134549.

The spelling of 21666191 in words is "twenty-one million, six hundred sixty-six thousand, one hundred ninety-one".