21664729 has 2 divisors, whose sum is σ = 21664730. Its totient is φ = 21664728.

The previous prime is 21664717. The next prime is 21664739. The reversal of 21664729 is 92746612.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 16999129 + 4665600 = 4123^2 + 2160^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21664729 is a prime.

It is not a weakly prime, because it can be changed into another prime (21664739) 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, 10832364 + 10832365.

It is an arithmetic number, because the mean of its divisors is an integer number (10832365).

Almost surely, 2^21664729 is an apocalyptic number.

It is an amenable number.

21664729 is a deficient number, since it is larger than the sum of its proper divisors (1).

21664729 is an equidigital number, since it uses as much as digits as its factorization.

21664729 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 36288, while the sum is 37.

The square root of 21664729 is about 4654.5385378145. The cubic root of 21664729 is about 278.7732427816.

The spelling of 21664729 in words is "twenty-one million, six hundred sixty-four thousand, seven hundred twenty-nine".