Dihedral groups Dn
Group of symmetries of a regular n-gon, consisting of n rotations and n reflections; order 2n,
generated by one rotation of order n and one reflection of order 2;
Dn = < g,h | gn=h2=1, hg=g-1h > = {e,g,g2,...,gn-1,h,gh,...,gn-1h}.
In other words, it is the split extension
Cn:C2 with C2
acting by -1.
(The non-split extension in this case is the dicyclic group.)
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