21305257 has 2 divisors, whose sum is σ = 21305258. Its totient is φ = 21305256.

The previous prime is 21305239. The next prime is 21305261. The reversal of 21305257 is 75250312.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 15848361 + 5456896 = 3981^2 + 2336^2 .

It is a cyclic number.

It is not a de Polignac number, because 21305257 - 2¹¹ = 21303209 is a prime.

It is not a weakly prime, because it can be changed into another prime (21305237) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10652628 + 10652629.

It is an arithmetic number, because the mean of its divisors is an integer number (10652629).

Almost surely, 2^21305257 is an apocalyptic number.

It is an amenable number.

21305257 is a deficient number, since it is larger than the sum of its proper divisors (1).

21305257 is an equidigital number, since it uses as much as digits as its factorization.

21305257 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 2100, while the sum is 25.

The square root of 21305257 is about 4615.7618006132. The cubic root of 21305257 is about 277.2227873118. Note that the first 3 decimals are identical.

Adding to 21305257 its reverse (75250312), we get a palindrome (96555569).

The spelling of 21305257 in words is "twenty-one million, three hundred five thousand, two hundred fifty-seven".