A revision example rearrangements and

A permutation is a rearrangement of a finite number of objects. For example, one permutation of the six letters a, b, c, d, e,f is to change a into b, b into c, c into a, to transpose d and e, and to leave/alone there are several ways of denoting this permutation. One way is to list the letters twice, once in the initial, natural order and again in the revised order ... 

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