Find the number of integers with so that is an integer multiple of 1001.
Solution
Note that , so the stated product must be a multiple of 7, as well as a multiple of 11, as well as a multiple of 13. There are 4 possible residues of modulo 11 for which the product is a multiple of 11; similarly, there are 4 possible residues of modulo 13 for which the product is a multiple of 13. However, there are only 3 possible residues of modulo 7 for which the product is a multiple of 7. Consider each of these possible triples of remainders. By the Chinese Remainder Theorem there is exactly one value of with achieving those remainders, and exactly one value of with achieving those remainders. Similarly, there is exactly one value of with with those same remainders. Hence there are 96 values of with such that is a multiple of 1001. It remains to check . Since the product must be a multiple of 7, we can narrow the set to . The first 3 values work trivially, since the product is 0. It can be easily checked that none of the remaining values of yield a product which is a multiple of 11. Hence, the final answer is .