15821 has 4 divisors (see below), whose sum is σ = 17052. Its totient is φ = 14592.

The previous prime is 15817. The next prime is 15823. The reversal of 15821 is 12851.

15821 = 11² + 12² + ... + 36².

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 12851 = 71 ⋅181.

It can be written as a sum of positive squares in 2 ways, for example, as 3721 + 12100 = 61^2 + 110^2 .

It is a cyclic number.

It is not a de Polignac number, because 15821 - 2² = 15817 is a prime.

It is a Duffinian number.

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 15821.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (15823) 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, 596 + ... + 621.

It is an arithmetic number, because the mean of its divisors is an integer number (4263).

2¹⁵⁸²¹ is an apocalyptic number.

It is an amenable number.

15821 is a deficient number, since it is larger than the sum of its proper divisors (1231).

15821 is a wasteful number, since it uses less digits than its factorization.

15821 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1230.

The product of its digits is 80, while the sum is 17.

The square root of 15821 is about 125.7815566766. The cubic root of 15821 is about 25.1040992654.

The spelling of 15821 in words is "fifteen thousand, eight hundred twenty-one".