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231 How Rules Work When I Calculate<br />

The square gets smaller or the square gets bigger. And this rule<br />

moves a triangle to the right, while its inverse<br />

does so to the left.<br />

But inverses needn’t be distinct when rules are defined in the schema x fi tðxÞ.<br />

An identity A fi A and its inverse are evidently the same, only this is trivial. Of greater<br />

interest, the rule<br />

that rotates a square at a corner, and its inverse<br />

are the same. You can check this in general using the transformation t A t 1 , where<br />

t A is a transformation in the symmetry group of x in x fi tðxÞ, and t Aðt 1 ðxÞÞ ¼ tðxÞ.

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