21606 has 16 divisors (see below), whose sum is σ = 46704. Its totient is φ = 6624.

The previous prime is 21601. The next prime is 21611. The reversal of 21606 is 60612.

21606 is nontrivially palindromic in base 5.

It is an interprime number because it is at equal distance from previous prime (21601) and next prime (21611).

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21601) 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 7 ways as a sum of consecutive naturals, for example, 61 + ... + 216.

It is an arithmetic number, because the mean of its divisors is an integer number (2919).

21606 is a Friedman number, since it can be written as 6*(60^2+1), using all its digits and the basic arithmetic operations.

2²¹⁶⁰⁶ is an apocalyptic number.

21606 is a gapful number since it is divisible by the number (26) formed by its first and last digit.

21606 is an abundant number, since it is smaller than the sum of its proper divisors (25098).

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 (23352).

21606 is a wasteful number, since it uses less digits than its factorization.

21606 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 295.

The product of its (nonzero) digits is 72, while the sum is 15.

The square root of 21606 is about 146.9897955642. The cubic root of 21606 is about 27.8521114234.

The spelling of 21606 in words is "twenty-one thousand, six hundred six".