How many functions satisfy the property that, for all ordered triples \left(a_{1}, a_{2}, a_{3}\right) and \left(b_{1}, b_{2}, b_{3}\right) such that for all ?
Solution
Consider the unit cube with vertices . Let , , and . We want to find a function on these vertices such that (and symmetric representations). For instance, if , then as well, and if , then as well. We group the vertices into four levels: , and . We do casework on the lowest level of a 1 in a function. - If the 1 is in , then maps everything to 1, for a total of 1 way. - If the 1 is in , then . If there are 31 's in , then everything but must be mapped to 1, for 1 way. If there are 21 's in , then , and there are 3 ways to choose the 21 's in , for a total of 3 ways. If there is one 1, then WLOG . Then , and equals either 0 or 1. There are ways to do this. In total, there are ways for the lowest 1 to be in . - If the lowest 1 is in , then . If there are 31 's in , there is one way to make . If there are 21 's, then we can pick the 21 's in 3 ways. Finally, if there is one 1, then we pick this 1 in 3 ways. There are ways. - The lowest 1 is in . There is 1 way. - There are no 1's. Then sends everything to 0. There is 1 way. In total, there are total 's.