81821 has 4 divisors (see below), whose sum is σ = 86652. Its totient is φ = 76992.

The previous prime is 81817. The next prime is 81839. The reversal of 81821 is 12818.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 62500 + 19321 = 250^2 + 139^2 .

It is a cyclic number.

It is not a de Polignac number, because 81821 - 2² = 81817 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is a junction number, because it is equal to n+sod(n) for n = 81793 and 81802.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (81421) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2390 + ... + 2423.

It is an arithmetic number, because the mean of its divisors is an integer number (21663).

2⁸¹⁸²¹ is an apocalyptic number.

It is an amenable number.

81821 is a deficient number, since it is larger than the sum of its proper divisors (4831).

81821 is a wasteful number, since it uses less digits than its factorization.

81821 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4830.

The product of its digits is 128, while the sum is 20.

The square root of 81821 is about 286.0437029546. The cubic root of 81821 is about 43.4131795433.

The spelling of 81821 in words is "eighty-one thousand, eight hundred twenty-one".