21645 has 24 divisors (see below), whose sum is σ = 41496. Its totient is φ = 10368.
The previous prime is 21617. The next prime is 21647. The reversal of 21645 is 54612.
21645 = T14 + T15 + ... + T50.
It is a happy number.
21645 is a `hidden beast` number, since 21 + 645 = 666.
It can be written as a sum of positive squares in 4 ways, for example, as 2601 + 19044 = 51^2 + 138^2 .
It is not a de Polignac number, because 21645 - 25 = 21613 is a prime.
It is a super-3 number, since 3×216453 = 30422438733375, which contains 333 as substring.
It is an Ulam number.
It is a Curzon number.
It is a nialpdrome in base 15.
It is a congruent number.
It is not an unprimeable number, because it can be changed into a prime (21647) 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 23 ways as a sum of consecutive naturals, for example, 567 + ... + 603.
It is an arithmetic number, because the mean of its divisors is an integer number (1729).
221645 is an apocalyptic number.
It is an amenable number.
21645 is a deficient number, since it is larger than the sum of its proper divisors (19851).
21645 is a wasteful number, since it uses less digits than its factorization.
21645 is an odious number, because the sum of its binary digits is odd.
The sum of its prime factors is 61 (or 58 counting only the distinct ones).
The product of its digits is 240, while the sum is 18.
The square root of 21645 is about 147.1223980229. The cubic root of 21645 is about 27.8688595401.
It can be divided in two parts, 21 and 645, that added together give a palindrome (666).
The spelling of 21645 in words is "twenty-one thousand, six hundred forty-five".
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