21605 has 8 divisors (see below), whose sum is σ = 27000. Its totient is φ = 16576.

The previous prime is 21601. The next prime is 21611. The reversal of 21605 is 50612.

21605 = T₄₆ + T₄₇ + ... + T₆₀.

It can be written as a sum of positive squares in 4 ways, for example, as 289 + 21316 = 17^2 + 146^2 .

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 21605 - 2² = 21601 is a prime.

It is a plaindrome in base 13.

It is a self number, because there is not a number n which added to its sum of digits gives 21605.

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, 71 + ... + 219.

It is an arithmetic number, because the mean of its divisors is an integer number (3375).

2²¹⁶⁰⁵ is an apocalyptic number.

It is an amenable number.

21605 is a deficient number, since it is larger than the sum of its proper divisors (5395).

21605 is a wasteful number, since it uses less digits than its factorization.

21605 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 183.

The product of its (nonzero) digits is 60, while the sum is 14.

The square root of 21605 is about 146.9863939281. The cubic root of 21605 is about 27.8516817196.

It can be divided in two parts, 21 and 605, that added together give a palindrome (626).

The spelling of 21605 in words is "twenty-one thousand, six hundred five".