Could someone explain Exercise

Find an operation on the set M = \{1, 2, 3, 4\}, i.e., a legitimate function f : M \times M \rightarrow M, such that f cannot be the multiplication formula for a monoid on M. That is, either it is not associative or no element of M can serve as a unit.


2 thoughts on “Exercise”

  1. Hi Max,

    You can assign a value for each pair of inputs in such a way that either it won’t have identity element or fails associativity.

    For example, if I take an elements (a,b) I can equate it to set {a,b}. Then the associativity (a*b)*c gets translated as (a*b) = (a,b) and *c will make it ((a,b),c). When we take a*(b*c), we get (a,(b,c)). Clearly associativity fails.

  2. Oops! Above example is incorrect as domain is not M. One can define a function such that f(2,1)=1 and f(3,1)=4 as well as f(3,2)=2 and f(2,1)=2.
    which shows associativity fails!

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