Sheng Li (gmachine1729) wrote,
Sheng Li

Automorphisms of quaternion group

Originally published at 狗和留美者不得入内. You can comment here or there.

I learned this morning from Brian Bi that the automorphism group of the quaternion group is in fact S_4. Why? The quaternion group is generated by any two of i,j,k all of which have order 4. \pm i, \pm j, \pm k correspond to the six faces of a cube. Remember that the symmetries orientation preserving of cube form S_4 with the objects permuted the space diagonals. Now what do the space diagonals correspond to? Triplet bases (i,j,k), (-i,j,-k), (j,i,-k), (-j,i,k), which correspond to four different corners of the cube, no two of which are joined by a space diagonal. We send both our generators i,j to two of \pm i, \pm j, \pm k; there are 6\cdot 4 = 24 choices. There are by the same logic 24 triplets (x,y,z) of quaternions such that xy = z. We define an equivalence relation with (x,y,z) \sim (-x,-y,z) and (x,y,z) \sim (y,z,x) \sim (z,x,y) that is such that if two elements are in the same equivalence class, then results of the application of any automorphism on those two elements will be as well. Furthermore, no two classes are mapped to the same class. Combined, this shows that every automorphism is a bijection on the equivalence classes.

Tags: algebra, group theory, 数学/математика

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