21664879 has 2 divisors, whose sum is σ = 21664880. Its totient is φ = 21664878.

The previous prime is 21664861. The next prime is 21664889. The reversal of 21664879 is 97846612.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 21664879 - 2¹⁷ = 21533807 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21664849) 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, 10832439 + 10832440.

It is an arithmetic number, because the mean of its divisors is an integer number (10832440).

Almost surely, 2^21664879 is an apocalyptic number.

21664879 is a deficient number, since it is larger than the sum of its proper divisors (1).

21664879 is an equidigital number, since it uses as much as digits as its factorization.

21664879 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 145152, while the sum is 43.

The square root of 21664879 is about 4654.5546510918. The cubic root of 21664879 is about 278.7738861605.

The spelling of 21664879 in words is "twenty-one million, six hundred sixty-four thousand, eight hundred seventy-nine".