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oeis.org/A063196 (odd numbers appearing twice)
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | ⋯
| | 0 | 1 | 3 | 3 | 5 | 5 | 7 | 7 | 9 | 9 | 11 | ⋯ |
Also, for $n>1$, number of involutions (i.e. elements of order 2) in the dihedral group $D_{n-1}$. - Lekraj Beedassy, Oct 22 2004
For odd $n$, involutions in $D_n$ are $s,rs,⋯,r^{n-1}s$, so number of involutions is $n$;
For even $n$, involutions in $D_n$ are $r^{n/2},s,rs,⋯,r^{n-1}s$, so number of involutions is $n+1$.
另一个小问题:
math.stackexchange.com/questions/160168/prove … olutions-is-dihedral
a group generated by two involutions is dihedral(由两个对合生成的群是二面体群) |
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