Never
spend so much time on teaching than this semester and never felt so
depressed afterwards. The final test for the first year course on
grouptheory (60 hrs. going from nothing to Jordan-Holder and the Sylow
theorems) included the following question :
Question :
For a subgroup $H \subset G $ define the normalizer to be the
subgroup $N_G(H) = \{ g \in G~:~gHg^{-1} = H \} $. Complete the
statement of the result for which the proof is given
below.
theorem : Let P be a Sylow subgroup of
a finite group G and suppose that H is a subgroup of G which
contains the normalizer $N_G(P) $. Then …
proof :
Let $u \in N_G(H) $. Now, $P \subset N_G(P) \subset H $
whence $uPu^{-1} \subset uHu^{-1} = H $. Thus, $uPu^{-1} $, being of the
same order as P is also a Sylow subgroup op H. Applying the Sylow
theorems to H we infer that there exists an element $h \in H $ such
that
$h(uPu^{-1})h^{-1} = P $. This means that $hu \in N_G(P) $.
Since, by hypotheses, $N_G(P) \subset H $, it follows that $hu \in H $.
As $h \in H $ it follows that $u \in H $, finishing the proof.
A
majority of the students was unable to do this… Sure, the result was
not contained in their course-notes (if it were I\’m certain all of them
would be able to give the correct statement as well as the full proof
by heart. It makes me wonder how much they understood
of the proof of the Sylow-theorems.) They (and others) blame it on the
fact that not every triviality is spelled out in my notes or on my
\’chaotic\’ teaching-style. I fear the real reason is contained in the
post-title…
But, I\’m still lucky to be working with students
who are interested in mathematics. I assume it can get a lot worse (but
also a lot funnier)
and what about this one :
If you are (like me) in urgent need for a smile, try out
this newsvine article for more
bloopers.
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