636821 has 2 divisors, whose sum is σ = 636822. Its totient is φ = 636820.

The previous prime is 636817. The next prime is 636829. The reversal of 636821 is 128636.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 555025 + 81796 = 745^2 + 286^2 .

It is a cyclic number.

It is not a de Polignac number, because 636821 - 2² = 636817 is a prime.

It is equal to p₅₁₈₅₇ and since 636821 and 51857 have the same sum of digits, it is a Honaker prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (636829) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 318410 + 318411.

It is an arithmetic number, because the mean of its divisors is an integer number (318411).

2⁶³⁶⁸²¹ is an apocalyptic number.

It is an amenable number.

636821 is a deficient number, since it is larger than the sum of its proper divisors (1).

636821 is an equidigital number, since it uses as much as digits as its factorization.

636821 is an evil number, because the sum of its binary digits is even.

The product of its digits is 1728, while the sum is 26.

The square root of 636821 is about 798.0106515580. The cubic root of 636821 is about 86.0344642829.

The spelling of 636821 in words is "six hundred thirty-six thousand, eight hundred twenty-one".