Question

For the positive integer $n,$ let $\langle n \rangle$ denote the sum of all the positive divisors of $n$ with the exception of $n$ itself. For example, $\langle 4 \rangle = 1 + 2 = 3$ and $\langle 12 \rangle = 1 + 2 + 3 + 4 + 6 = 16.$ What is $\langle\langle\langle 6 \rangle\rangle\rangle?$

$6$

$12$

$24$

$32$

$36$

Solution:

The positive divisors of $6$ other than $6$ are $1, 2,$ and $3,$ so $\langle 6 \rangle = 1 + 2 + 3 = 6.$

Since applying the operation to $6$ again returns $6,$ we get $\langle\langle\langle 6 \rangle\rangle\rangle = 6.$

(A number equal to the sum of its proper divisors is called a perfect number, and $6$ is the smallest.)

Thus, the correct answer is A.

2008 AMC 10A Problem 3

Solution:

Problem 3 in Other Years