21666397 has 2 divisors, whose sum is σ = 21666398. Its totient is φ = 21666396.

The previous prime is 21666383. The next prime is 21666443. The reversal of 21666397 is 79366612.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 21631801 + 34596 = 4651^2 + 186^2 .

It is a cyclic number.

It is not a de Polignac number, because 21666397 - 2¹⁹ = 21142109 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 (21666317) 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, 10833198 + 10833199.

It is an arithmetic number, because the mean of its divisors is an integer number (10833199).

Almost surely, 2^21666397 is an apocalyptic number.

It is an amenable number.

21666397 is a deficient number, since it is larger than the sum of its proper divisors (1).

21666397 is an equidigital number, since it uses as much as digits as its factorization.

21666397 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 81648, while the sum is 40.

The square root of 21666397 is about 4654.7177143195. The cubic root of 21666397 is about 278.7803969881.

The spelling of 21666397 in words is "twenty-one million, six hundred sixty-six thousand, three hundred ninety-seven".