21681257 has 4 divisors (see below), whose sum is σ = 23349060. Its totient is φ = 20013456.

The previous prime is 21681221. The next prime is 21681269. The reversal of 21681257 is 75218612.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 10896601 + 10784656 = 3301^2 + 3284^2 .

It is a cyclic number.

It is not a de Polignac number, because 21681257 - 2⁶ = 21681193 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21681217) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 833882 + ... + 833907.

It is an arithmetic number, because the mean of its divisors is an integer number (5837265).

Almost surely, 2^21681257 is an apocalyptic number.

It is an amenable number.

21681257 is a deficient number, since it is larger than the sum of its proper divisors (1667803).

21681257 is a wasteful number, since it uses less digits than its factorization.

21681257 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1667802.

The product of its digits is 6720, while the sum is 32.

The square root of 21681257 is about 4656.3136707056. The cubic root of 21681257 is about 278.8441167038.

Adding to 21681257 its reverse (75218612), we get a palindrome (96899869).

The spelling of 21681257 in words is "twenty-one million, six hundred eighty-one thousand, two hundred fifty-seven".