For positive integers , let be the smallest positive integer for which is divisible by 210, if such a positive integer exists, and otherwise. What is ?
Solution
In order for , we must have , so we need only consider such . The number is divisible by 210 iff it is divisible by each of 2, 3, 5, and 7, and we can consider the order of modulo each modulus separately; will simply be the LCM of these orders. We can ignore the modulus 2 because order is always 1. For the other moduli, the sets of orders are , , . By the Chinese Remainder Theorem, each triplet of choices from these three multisets occurs for exactly one in the range , so the answer we seek is the sum of over in the Cartesian product of these multisets. For this table of LCMs is as follows: which has a sum of . The table for is identical except for the top row, where are replaced by , and thus has a total sum of 7 more, or 168. So our answer is .