21138257 has 12 divisors (see below), whose sum is σ = 24664356. Its totient is φ = 18063360.

The previous prime is 21138253. The next prime is 21138277. The reversal of 21138257 is 75283112.

It can be written as a sum of positive squares in 2 ways, for example, as 13608721 + 7529536 = 3689^2 + 2744^2 .

It is not a de Polignac number, because 21138257 - 2² = 21138253 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21138251) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 31073 + ... + 31745.

It is an arithmetic number, because the mean of its divisors is an integer number (2055363).

Almost surely, 2^21138257 is an apocalyptic number.

It is an amenable number.

21138257 is a deficient number, since it is larger than the sum of its proper divisors (3526099).

21138257 is an equidigital number, since it uses as much as digits as its factorization.

21138257 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1328 (or 1321 counting only the distinct ones).

The product of its digits is 3360, while the sum is 29.

The square root of 21138257 is about 4597.6360230014. The cubic root of 21138257 is about 276.4965549963.

The spelling of 21138257 in words is "twenty-one million, one hundred thirty-eight thousand, two hundred fifty-seven".