20821 has 4 divisors (see below), whose sum is σ = 21312. Its totient is φ = 20332.

The previous prime is 20809. The next prime is 20849. The reversal of 20821 is 12802.

20821 = T₃ + T₄ + ... + T₄₉.

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 a cyclic number.

It is not a de Polignac number, because 20821 - 2⁵ = 20789 is a prime.

It is a Duffinian number.

20821 is a lucky number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (20021) 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, 175 + ... + 268.

It is an arithmetic number, because the mean of its divisors is an integer number (5328).

2²⁰⁸²¹ is an apocalyptic number.

It is an amenable number.

20821 is a deficient number, since it is larger than the sum of its proper divisors (491).

20821 is an equidigital number, since it uses as much as digits as its factorization.

20821 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 490.

The product of its (nonzero) digits is 32, while the sum is 13.

The square root of 20821 is about 144.2948370525. The cubic root of 20821 is about 27.5106294997.

It can be divided in two parts, 20 and 821, that added together give a square (841 = 29²).

The spelling of 20821 in words is "twenty thousand, eight hundred twenty-one".