Find Z(D2n), where D2n is the dihedral group with 2n elements,
i) when n is an odd integer;
ii) when n is an even integer.
i)
ii)
r and s generate D2n with the group presentation
Proof:
i)
For any , x must commute with both s and r, since both are in D2n. If x
commutes with both s and r, then x commutes with any element of D2n since r and s generate D2n.
Then for any , we have xr=rx, where x is of the form x=sjrw. Since r and s have finite order, the exponents are modulo n and modulo 2 for rand s respectively.
Then we have sjrwr=rsjrw sjrw+1=sjrw-1
In the case where the power of r on the RHS is w-1, it is clear we cannot have equality unless
|r|=1, which it is not. If j is even then we have only the w+1 case. Hence x may be of the form
1rw=rw.
Also we have that x must commute with s, hence sx=xs.
Then srw=rws srw=sr-w
Applying s-1 to both sides we arrive at the task of finding when rw=r-w i.e when w=-w. But since r has finite order we have:
where is the residue class of w modulo n. Hence for some
w+an=wn-w
2w=wn-an
2w=n(w-a)
Since 0<w<n we have that the LHS is greater than 0. Also since then ,
since otherwise the RHS would be greater than the LHS. So since n is odd we must have that
(w-a) is even and hence (w-a)=2, implying that 2w=2n and hence n=w.
Hence x=rw=rn=1. So Z(D_{2n})=1
ii)
From the argument above any must be of the form sjrw for j even and .
Then x is of the form rw for . Since n is even we have the equation below is satisfied when 2w=n or w=n, since but also for
2w=n(w-a)
Hence when 2w=n, as desired.
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