21006257 has 2 divisors, whose sum is σ = 21006258. Its totient is φ = 21006256.

The previous prime is 21006247. The next prime is 21006259. The reversal of 21006257 is 75260012.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 16516096 + 4490161 = 4064^2 + 2119^2 .

It is a cyclic number.

It is not a de Polignac number, because 21006257 - 2²⁰ = 19957681 is a prime.

Together with 21006259, it forms a pair of twin primes.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (21006259) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10503128 + 10503129.

It is an arithmetic number, because the mean of its divisors is an integer number (10503129).

Almost surely, 2^21006257 is an apocalyptic number.

It is an amenable number.

21006257 is a deficient number, since it is larger than the sum of its proper divisors (1).

21006257 is an equidigital number, since it uses as much as digits as its factorization.

21006257 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 840, while the sum is 23.

The square root of 21006257 is about 4583.2583387804. The cubic root of 21006257 is about 275.9198158514.

Adding to 21006257 its reverse (75260012), we get a palindrome (96266269).

The spelling of 21006257 in words is "twenty-one million, six thousand, two hundred fifty-seven".