821681 has 12 divisors (see below), whose sum is σ = 981540. Its totient is φ = 685440.

The previous prime is 821677. The next prime is 821741. The reversal of 821681 is 186128.

It can be written as a sum of positive squares in 2 ways, for example, as 379456 + 442225 = 616^2 + 665^2 .

It is not a de Polignac number, because 821681 - 2² = 821677 is a prime.

It is a super-2 number, since 2×821681² = 1350319331522, which contains 22 as substring.

It is a Curzon number.

It is a self number, because there is not a number n which added to its sum of digits gives 821681.

It is not an unprimeable number, because it can be changed into a prime (821641) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 1805 + ... + 2213.

It is an arithmetic number, because the mean of its divisors is an integer number (81795).

2⁸²¹⁶⁸¹ is an apocalyptic number.

It is an amenable number.

821681 is a deficient number, since it is larger than the sum of its proper divisors (159859).

821681 is a wasteful number, since it uses less digits than its factorization.

821681 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 464 (or 457 counting only the distinct ones).

The product of its digits is 768, while the sum is 26.

The square root of 821681 is about 906.4662155867. The cubic root of 821681 is about 93.6629319033.

The spelling of 821681 in words is "eight hundred twenty-one thousand, six hundred eighty-one".