Find all positive integers for which simultaneously divides , divides , and divides .
Solution
The values of with this property are for . First, note that divides if and only if is itself a power of 2; we may thus write and note that if , then Moreover, the case does not lead to a solution because for , does not divide ; we may thus assume . Next, note that modulo , the powers of cycle with period (the terms remain the same upon reduction, and then the next term repeats the initial 1); consequently, divides if and only if divides , which happens if and only if is a power of 2. Write and note that , so . The case does not lead to a solution because for , does not divide ; we may thus assume . Finally, note that divides if and only if divides . By the same logic as the previous paragraph, this happens if and only if divides , that is, if divides . This in turn happens if and only if divides , which happens if and only if is a power of 2. The values allowed by the bound are ; for these values, and so the solutions listed do satisfy the original inequality.