1821 has 4 divisors (see below), whose sum is σ = 2432. Its totient is φ = 1212.

The previous prime is 1811. The next prime is 1823. The reversal of 1821 is 1281.

It is a happy number.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is not a de Polignac number, because 1821 - 2⁵ = 1789 is a prime.

It is a D-number.

It is a Duffinian number.

It is a Curzon number.

It is a plaindrome in base 6.

It is a nialpdrome in base 3, base 7, base 13 and base 14.

It is a junction number, because it is equal to n+sod(n) for n = 1797 and 1806.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (1823) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 301 + ... + 306.

It is an arithmetic number, because the mean of its divisors is an integer number (608).

2¹⁸²¹ is an apocalyptic number.

It is an amenable number.

1821 is a deficient number, since it is larger than the sum of its proper divisors (611).

1821 is an equidigital number, since it uses as much as digits as its factorization.

1821 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 610.

The product of its digits is 16, while the sum is 12.

The square root of 1821 is about 42.6731765867. The cubic root of 1821 is about 12.2115272220.

It can be divided in two parts, 18 and 21, that multiplied together give a triangular number (378 = T₂₇).

The spelling of 1821 in words is "one thousand, eight hundred twenty-one".