21664081 has 2 divisors, whose sum is σ = 21664082. Its totient is φ = 21664080.

The previous prime is 21664067. The next prime is 21664109. The reversal of 21664081 is 18046612.

It is an a-pointer prime, because the next prime (21664109) can be obtained adding 21664081 to its sum of digits (28).

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 14561856 + 7102225 = 3816^2 + 2665^2 .

It is a cyclic number.

It is not a de Polignac number, because 21664081 - 2¹³ = 21655889 is a prime.

It is a super-2 number, since 2×21664081² = 938664811149122, which contains 22 as substring.

It is not a weakly prime, because it can be changed into another prime (21664051) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10832040 + 10832041.

It is an arithmetic number, because the mean of its divisors is an integer number (10832041).

Almost surely, 2^21664081 is an apocalyptic number.

It is an amenable number.

21664081 is a deficient number, since it is larger than the sum of its proper divisors (1).

21664081 is an equidigital number, since it uses as much as digits as its factorization.

21664081 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 2304, while the sum is 28.

The square root of 21664081 is about 4654.4689278155. The cubic root of 21664081 is about 278.7704633506.

The spelling of 21664081 in words is "twenty-one million, six hundred sixty-four thousand, eighty-one".