21204251 has 2 divisors, whose sum is σ = 21204252. Its totient is φ = 21204250.

The previous prime is 21204241. The next prime is 21204329. The reversal of 21204251 is 15240212.

It is a weak prime.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21204251 is a prime.

It is a Sophie Germain prime.

It is not a weakly prime, because it can be changed into another prime (21204241) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10602125 + 10602126.

It is an arithmetic number, because the mean of its divisors is an integer number (10602126).

Almost surely, 2^21204251 is an apocalyptic number.

21204251 is a deficient number, since it is larger than the sum of its proper divisors (1).

21204251 is an equidigital number, since it uses as much as digits as its factorization.

21204251 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 160, while the sum is 17.

The square root of 21204251 is about 4604.8073792505. The cubic root of 21204251 is about 276.7839983942.

Adding to 21204251 its reverse (15240212), we get a palindrome (36444463).

The spelling of 21204251 in words is "twenty-one million, two hundred four thousand, two hundred fifty-one".