636721321 has 2 divisors, whose sum is σ = 636721322. Its totient is φ = 636721320.

The previous prime is 636721313. The next prime is 636721373. The reversal of 636721321 is 123127636.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 436810000 + 199911321 = 20900^2 + 14139^2 .

It is a cyclic number.

It is not a de Polignac number, because 636721321 - 2³ = 636721313 is a prime.

It is not a weakly prime, because it can be changed into another prime (636721391) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 318360660 + 318360661.

It is an arithmetic number, because the mean of its divisors is an integer number (318360661).

Almost surely, 2^636721321 is an apocalyptic number.

It is an amenable number.

636721321 is a deficient number, since it is larger than the sum of its proper divisors (1).

636721321 is an equidigital number, since it uses as much as digits as its factorization.

636721321 is an evil number, because the sum of its binary digits is even.

The product of its digits is 9072, while the sum is 31.

The square root of 636721321 is about 25233.3374922938. The cubic root of 636721321 is about 860.2997517384.

Adding to 636721321 its reverse (123127636), we get a palindrome (759848957).

The spelling of 636721321 in words is "six hundred thirty-six million, seven hundred twenty-one thousand, three hundred twenty-one".