Question

In: Advanced Math

Prove that each element in Pentagon D5 has a unique inverse under the binary operation. D5={AF,...

Prove that each element in Pentagon D5 has a unique inverse under the binary operation.

D5={AF, BF, CF, DF, EF,0,72,144,216,288}

Solutions

Expert Solution

Solution:

Set of symmetries of an regular pentagon with vertices levelled as in anticlockwise order. There are rotations ( ) where means rotation about degrees and reflections    () where means reflection about the vertices .

Observe that

  • We can consider each rotations and reflections as a bijective function. For example in ,If we take an reflection say ,then
  • rotation about zero degree. this means there is no change in the original figure, so is treated as an identity function.
  • since we saw that each element of can be treated as a bijective function, so it is obvious that we can take binary operation as composition of function.
  • [This example shows you how the operation works.]

We can prove the statement through Cayley Table.

From the Cayley table one can say that the inverse element of that is , similarly and . Here from the Cayley table we conclude that each element of has unique inverse element. hence proved.


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