Make a list of all irreducible polynomials of degrees 1 to 5 over F2

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Discussion Overview

The discussion revolves around identifying all irreducible polynomials of degrees 1 to 5 over the finite field F2. Participants explore various solutions and methods for determining irreducibility, as well as addressing discrepancies with a textbook solution.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants propose that the polynomial ##X^5+X^4+X^2+X+1## is a valid degree 5 irreducible polynomial, suggesting it may have been omitted from the textbook's solution.
  • Others agree with the inclusion of ##X^5+X^4+X^2+X+1## and also mention additional degree 5 polynomials: ##X^5+X^4+X^3+X+1##, ##X^5+X^3+X^2+X+1##, and ##X^5+X^4+X^3+X^2+1##.
  • One participant discusses the criteria for irreducibility in polynomials over Z/2, noting that irreducible cubics must end in 1 and have an odd number of terms, providing examples such as ##X^3+X^2+1## and ##X^3+X+1##.
  • There is mention of the tedious nature of checking irreducibility and references to broader challenges in algebraic geometry related to irreducibility.

Areas of Agreement / Disagreement

Participants express differing views on the completeness of the textbook's solution, with some asserting that additional irreducible polynomials exist while others seem to agree on the identified polynomials. The discussion remains unresolved regarding the totality of irreducible polynomials of degree 5.

Contextual Notes

Participants note that checking irreducibility can be inherently difficult, and there are references to unresolved mathematical steps and criteria for irreducibility that may depend on specific definitions.

Hill
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TL;DR
F2 = (0, 1)
To this exercise,
1735996974647.png

the textbook's solution is
1735997016225.png

I didn't follow their method and have found another degree 5 solution, ##X^5+X^4+X^2+X+1##.
Is it wrong or the book has omitted this solution?
 
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Hill said:
TL;DR Summary: F2 = (0, 1)

To this exercise,
View attachment 355323
the textbook's solution is
View attachment 355324
I didn't follow their method and have found another degree 5 solution, ##X^5+X^4+X^2+X+1##.
Is it wrong or the book has omitted this solution?
It is irreducible. I guess they missed it.
 
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martinbn said:
It is irreducible. I guess they missed it.
As well as
##X^5+X^4+X^3+X+1##
##X^5+X^3+X^2+X+1##
##X^5+X^4+X^3+X^2+1##
(?)
 
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Hill said:
As well as
##X^5+X^4+X^3+X+1##
##X^5+X^3+X^2+X+1##
##X^5+X^4+X^3+X^2+1##
(?)
Yes.
There are six of them.
 
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This seems a little easier than the book's hints make it. A poly. over Z/2 is divisible by X or X+1 iff it either ends in X or has an even number of terms, hence the irreducible cubics end in 1 and have an odd number of terms, i.e. X^3+X^2 + 1, and X^3+X+1. A reducible quartic or quintic either ends in X or has an even number of terms or is divisible by X^2+X+1....It is still somewhat tedious, but maybe not as long as multiplying together all those factors to compute all reducible examples. It is interesting that after all these years I still have trouble checking irreducibility. I guess it is just inherently difficult. Such problems have been challenging for years in algebraic geometry: Severi's conjecture, irreducibility of moduli of curves, components of Hilbert schemes,....
 

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