21581257 has 2 divisors, whose sum is σ = 21581258. Its totient is φ = 21581256.

The previous prime is 21581233. The next prime is 21581267. The reversal of 21581257 is 75218512.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 16491721 + 5089536 = 4061^2 + 2256^2 .

It is a cyclic number.

It is not a de Polignac number, because 21581257 - 2¹¹ = 21579209 is a prime.

It is not a weakly prime, because it can be changed into another prime (21581267) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10790628 + 10790629.

It is an arithmetic number, because the mean of its divisors is an integer number (10790629).

Almost surely, 2^21581257 is an apocalyptic number.

It is an amenable number.

21581257 is a deficient number, since it is larger than the sum of its proper divisors (1).

21581257 is an equidigital number, since it uses as much as digits as its factorization.

21581257 is an evil number, because the sum of its binary digits is even.

The product of its digits is 5600, while the sum is 31.

The square root of 21581257 is about 4645.5631520839. The cubic root of 21581257 is about 278.4147536510.

Adding to 21581257 its reverse (75218512), we get a palindrome (96799769).

The spelling of 21581257 in words is "twenty-one million, five hundred eighty-one thousand, two hundred fifty-seven".