Archive for the ‘symmetry’ Category

4!-torsor a la George Hart

20 April 2015

As a project with a certain 4-year-old relative of mine, we constructed the proof I described before that the outer vertices of George Hart’s 12-Card Star form a 4!-torsor.  (I guess I didn’t say it that way before, but it’s true!)  Here’s our proof:

IMG_2215Last time I suggested using a deck of 12 cards like this:

deckoftwosBut instead, we used four solid colors, three cards of each.  So, our “star” permutes the colors red, white, black, and silver:


You can get any permutation of these colors in our Star by exactly one symmetry taking outer vertices to outer vertices.  The “exactly one” in this isomorphism is what makes the set of outer vertices a 4!-torsor rather than just a 4!-set.

Here’s what it looks like when you put three pieces together, from both sides: