21666811 has 2 divisors, whose sum is σ = 21666812. Its totient is φ = 21666810.

The previous prime is 21666791. The next prime is 21666817. The reversal of 21666811 is 11866612.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 21666811 - 2⁵ = 21666779 is a prime.

It is not a weakly prime, because it can be changed into another prime (21666817) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10833405 + 10833406.

It is an arithmetic number, because the mean of its divisors is an integer number (10833406).

Almost surely, 2^21666811 is an apocalyptic number.

21666811 is a deficient number, since it is larger than the sum of its proper divisors (1).

21666811 is an equidigital number, since it uses as much as digits as its factorization.

21666811 is an evil number, because the sum of its binary digits is even.

The product of its digits is 3456, while the sum is 31.

The square root of 21666811 is about 4654.7621851175. The cubic root of 21666811 is about 278.7821726156.

The spelling of 21666811 in words is "twenty-one million, six hundred sixty-six thousand, eight hundred eleven".