840821 has 2 divisors, whose sum is σ = 840822. Its totient is φ = 840820.

The previous prime is 840817. The next prime is 840823. The reversal of 840821 is 128048.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 703921 + 136900 = 839^2 + 370^2 .

It is a cyclic number.

It is not a de Polignac number, because 840821 - 2² = 840817 is a prime.

It is a super-3 number, since 3×840821³ = 1783330775810122983, which contains 333 as substring.

Together with 840823, it forms a pair of twin primes.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (840823) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 420410 + 420411.

It is an arithmetic number, because the mean of its divisors is an integer number (420411).

2⁸⁴⁰⁸²¹ is an apocalyptic number.

It is an amenable number.

840821 is a deficient number, since it is larger than the sum of its proper divisors (1).

840821 is an equidigital number, since it uses as much as digits as its factorization.

840821 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 512, while the sum is 23.

The square root of 840821 is about 916.9629218240. The cubic root of 840821 is about 94.3846094918.

Adding to 840821 its reverse (128048), we get a palindrome (968869).

It can be divided in two parts, 840 and 821, that added together give a palindrome (1661).

The spelling of 840821 in words is "eight hundred forty thousand, eight hundred twenty-one".