21664861 has 2 divisors, whose sum is σ = 21664862. Its totient is φ = 21664860.

The previous prime is 21664849. The next prime is 21664879. The reversal of 21664861 is 16846612.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 20702500 + 962361 = 4550^2 + 981^2 .

It is a cyclic number.

It is not a de Polignac number, because 21664861 - 2¹¹ = 21662813 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21664801) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10832430 + 10832431.

It is an arithmetic number, because the mean of its divisors is an integer number (10832431).

Almost surely, 2^21664861 is an apocalyptic number.

It is an amenable number.

21664861 is a deficient number, since it is larger than the sum of its proper divisors (1).

21664861 is an equidigital number, since it uses as much as digits as its factorization.

21664861 is an evil number, because the sum of its binary digits is even.

The product of its digits is 13824, while the sum is 34.

The square root of 21664861 is about 4654.5527175014. The cubic root of 21664861 is about 278.7738089552.

The spelling of 21664861 in words is "twenty-one million, six hundred sixty-four thousand, eight hundred sixty-one".