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Polynomials/ galois field question 
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#1
Dec1807, 02:32 AM

P: 140

Im reading through a section that deals with polynomials Galois fields and ran into something that Im not quite understanding.
Say we have an irreducible polynomial, f(x), which has coefficients from GF(2) and roots [tex]\beta[/tex], [tex]\beta[/tex][tex]^{2}[/tex], [tex]\beta[/tex][tex]^{4}[/tex], [tex]\beta[/tex][tex]^{8}[/tex], .........[tex]\beta[/tex][tex]^{2}[/tex][tex]^{e}[/tex][tex]1[/tex] where e is the smallest integer such that [tex]\beta[/tex][tex]^{2}[/tex][tex]^{e}[/tex] = [tex]\beta[/tex] given by f(x) = [tex]\prod[/tex][tex]^{e1}_{i=0}[/tex] ( X + [tex]\beta[/tex][tex]^{2^i}[/tex]) Note: Beta term is Beta^(2^i) In the section Im reading, they do a test to prove f(x) is irreducible. I will state the test below Say f(x) = a(x).b(x) where a(x) and b(x) are polynomials with coefficients from GF(2) if we sub one of the roots of f(x) in, say [tex]\beta[/tex], f([tex]\beta[/tex]) = 0 which means that either a([tex]\beta[/tex]) = 0 or b([tex]\beta[/tex]) = 0, hence a(x) = f(x) or b(x) = f(x). This understanding also says that a(x) or b(x) (depending which one had [tex]\beta[/tex] subbed into it) has all the roots of f(x) (A theory in my textbook says that if f([tex]\beta[/tex]) = 0, f([tex]\beta[/tex][tex]^{2^i}[/tex])=0 for any i) I get how they arrive at their result, however Im still clueless as to how this proves that f(x) is irreducible. insight is appreciated regards James 


#2
Dec1807, 04:04 AM

P: 140

Ok I read over the notes again and think I may have the answer
Since f(x) = a(x) or f(x) = b(x) when the root is substituted in, it cannot be divided into a smaller polynomial with a nonzero degree. Therefore f(x) must be irreducible. Thoughts, comments, insights ?? 


#3
Dec1807, 10:54 AM

Sci Advisor
HW Helper
P: 2,020

I'm confused. You start out by saying that f(x) is irreducible, presumably over GF(2). Then you go on to prove that f(x) is irreducible  this time over what?
Also, why is this in the Set Theory, Logic, Probability, Statistics forum? It really should be in the algebra forum. 


#4
Dec1907, 01:52 AM

P: 140

Polynomials/ galois field question
Sorry about the confusion Also, someone pls move this topic to the correct forum. ta 


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