21810 has 16 divisors (see below), whose sum is σ = 52416. Its totient is φ = 5808.

The previous prime is 21803. The next prime is 21817. The reversal of 21810 is 1812.

It is a happy number.

21810 is nontrivially palindromic in base 9.

It is an interprime number because it is at equal distance from previous prime (21803) and next prime (21817).

It is a super-2 number, since 2×21810² = 951352200, which contains 22 as substring.

It is an alternating number because its digits alternate between even and odd.

It is a nialpdrome in base 16.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 21810.

It is not an unprimeable number, because it can be changed into a prime (21817) by changing a digit.

21810 is an untouchable number, because it is not equal to the sum of proper divisors of any number.

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 7 ways as a sum of consecutive naturals, for example, 334 + ... + 393.

It is an arithmetic number, because the mean of its divisors is an integer number (3276).

2²¹⁸¹⁰ is an apocalyptic number.

21810 is an abundant number, since it is smaller than the sum of its proper divisors (30606).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

It is a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (26208).

21810 is a wasteful number, since it uses less digits than its factorization.

21810 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 737.

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

The square root of 21810 is about 147.6820909928. The cubic root of 21810 is about 27.9394952006.

The spelling of 21810 in words is "twenty-one thousand, eight hundred ten".