21666247 has 2 divisors, whose sum is σ = 21666248. Its totient is φ = 21666246.

The previous prime is 21666209. The next prime is 21666263. The reversal of 21666247 is 74266612.

It is a strong prime.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21666247 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21666287) 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, 10833123 + 10833124.

It is an arithmetic number, because the mean of its divisors is an integer number (10833124).

Almost surely, 2^21666247 is an apocalyptic number.

21666247 is a deficient number, since it is larger than the sum of its proper divisors (1).

21666247 is an equidigital number, since it uses as much as digits as its factorization.

21666247 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 24192, while the sum is 34.

The square root of 21666247 is about 4654.7016016067. The cubic root of 21666247 is about 278.7797536393.

The spelling of 21666247 in words is "twenty-one million, six hundred sixty-six thousand, two hundred forty-seven".