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constructible angles with integer values in degrees

constructible angle
Type of Math Object: 
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Mathematics Subject Classification

11S20 no label found11R32 no label found51M15 no label found13B05 no label found


where is the proof that the angle of 20 degrees can not be constructed?

in pretty much any galois theory book

ok a nicer reply:
the proof relies on 20º not being constructible.
This is a corollary from the proof that
60º can't be trisected
which is the STANDARD proof that
the trisection of angles is impossible

so.. I'm relying in a known fact
everytime yo prove something you take some things for granted

I could add it, but it has a completely different
(and non elementary) context, so I left it out as known fact

And realize this is always done, If I had written the actual proof
would you request to add inside the proof to the galois theory results
it uses? and then the proof the the results used on those proof and ad infinitum?
G -----> H G
p \ /_ ----- ~ f(G)
\ / f ker f
G/ker f

Well, it really would be nice if "trisection of the angle is impossible" were an entry in PM, possibly along with doubling the cube and squaring the circle; all three could be examples somethere in Galois-theory-land.

As for needing proofs for all the Galois theory results, that's a problem for the entries on the actual results --- but it makes sense to hope that those results actually have entries.

Yes, that's what I meant

The style and techniques used in proving the trisection of angle don't go very well with this entry, I try to keep things in a n elementary level as possible

So yes, a "trisecting angle" entry should be added (with sysnonyms mentioning impossibility at all, it could even become a topic and then we could attach this entry and several others to it
G -----> H G
p \ /_ ----- ~ f(G)
\ / f ker f
G/ker f

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