21515257 has 2 divisors, whose sum is σ = 21515258. Its totient is φ = 21515256.

The previous prime is 21515233. The next prime is 21515261. The reversal of 21515257 is 75251512.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 21427641 + 87616 = 4629^2 + 296^2 .

It is a cyclic number.

It is not a de Polignac number, because 21515257 - 2¹¹ = 21513209 is a prime.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (21515227) 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, 10757628 + 10757629.

It is an arithmetic number, because the mean of its divisors is an integer number (10757629).

Almost surely, 2^21515257 is an apocalyptic number.

It is an amenable number.

21515257 is a deficient number, since it is larger than the sum of its proper divisors (1).

21515257 is an equidigital number, since it uses as much as digits as its factorization.

21515257 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 3500, while the sum is 28.

The square root of 21515257 is about 4638.4541606014. The cubic root of 21515257 is about 278.1306469754.

Adding to 21515257 its reverse (75251512), we get a palindrome (96766769).

The spelling of 21515257 in words is "twenty-one million, five hundred fifteen thousand, two hundred fifty-seven".