21366721 has 2 divisors, whose sum is σ = 21366722. Its totient is φ = 21366720.

The previous prime is 21366713. The next prime is 21366727. The reversal of 21366721 is 12766312.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 13727025 + 7639696 = 3705^2 + 2764^2 .

It is a cyclic number.

It is not a de Polignac number, because 21366721 - 2³ = 21366713 is a prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21366721.

It is not a weakly prime, because it can be changed into another prime (21366727) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10683360 + 10683361.

It is an arithmetic number, because the mean of its divisors is an integer number (10683361).

Almost surely, 2^21366721 is an apocalyptic number.

It is an amenable number.

21366721 is a deficient number, since it is larger than the sum of its proper divisors (1).

21366721 is an equidigital number, since it uses as much as digits as its factorization.

21366721 is an evil number, because the sum of its binary digits is even.

The product of its digits is 3024, while the sum is 28.

The square root of 21366721 is about 4622.4150614154. The cubic root of 21366721 is about 277.4891200453.

The spelling of 21366721 in words is "twenty-one million, three hundred sixty-six thousand, seven hundred twenty-one".