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   Author  Topic: tough number theory prob.  (Read 2929 times)
Grimbal
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Re: tough number theory prob.  
« Reply #25 on: Oct 18th, 2008, 11:54pm »
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The way I see it is that the correct notation is:
   a b (mod n)
and it should be understood as
   a b  (please understand as the equivalence modulo n)
So, "(mod n)" is not part of the expression, but gives a context to it.
 
The equivalence
   a b  (mod n)
means  
   a = b + k·n   for some k
which implies
   rem(a,n) = rem(b,n)
 
This has nothing to do with the binary operator 'mod' that you can see in some computer languages.
 
In mathematics you use " ... (mod n)" equivalence and express the remainder with a rem(x,n) function, while in some computer languages, or calculators, you use "mod" as an operator to compute a remainder.
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Michael Dagg
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Re: tough number theory prob.  
« Reply #26 on: Nov 7th, 2008, 12:23pm »
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Nice discussion and as pointed out, this is an interesting  
problem as well as ones like it. It is crucial in many problems  
to know the group of units in an algebraic number field.  
(For example, when "ideals", then viewed as "ideal numbers" were  
first invented by Kummer, it was in the context of proving Fermat's  
Last Theorem. He got as far as calculating that some important  
ideal would be principal, but then had to wrestle with the possible  
ways that the generator could be presented; the options differ  
from one another by multiplication by a unit. So it was very  
important to know the group of units in those algebraic number  
fields.)
 
A summary of what happens is that the group of units in the  
ring of integers in an algebraic number field  F  is a  
finitely-generated abelian group, (that's not obvious!) hence (easy)  
it's isomorphic to a finite group  T  times a free abelian group  Z^r .  
The finite subgroup  T  is (easy) the group of roots of unity that  
lie in the field, and that' not usually hard to determine; at worst  
you have to try factoring a finite list of cyclotomic polynomials  
like  x^n - 1  over the field. The torsion-free part can be shown  
to be of rank  s1+s2  where  s1  is the number of embeddings of the  
field into the real numbers, and  s2  is the number of complex-conjugate  
pairs of non-real embeddings. (These numbers make s1 + 2 s2 = [ F : Q ] .)  
The calculation of this value of  r  is a little bit clever but not  
hard to follow. The hardest part of all is to actually compute a set  
of  r  generators of this group; typically one can find a set of  r  
independent units, i.e. one can find a group of units which is of  
finite index in the full group of units, but then you have to decide  
whether for example any simple combination of the units you know is  
the square of another unit you didn't previously know about.  
 
I like the book Number Theory by Borevich and Shafarevich (despite its
numerous misprints and lousy index) because it treats all these topics.
Definitely one to add to your bookshelf.
« Last Edit: Nov 7th, 2008, 12:24pm by Michael Dagg » IP Logged

Regards,
Michael Dagg
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