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)abbrev package NORMRETR NormRetractPackage
++ Description:
++ This package has no description

NormRetractPackage(F, ExtF, SUEx, ExtP, n) : SIG  == CODE where
  F : FiniteFieldCategory
  ExtF : FiniteAlgebraicExtensionField(F)
  SUEx : UnivariatePolynomialCategory ExtF
  ExtP : UnivariatePolynomialCategory SUEx
  n : PositiveInteger

  SUP       ==>  SparseUnivariatePolynomial
  R         ==>  SUP F
  P         ==>  SUP R

  SIG ==> with

    normFactors : ExtP -> List ExtP
      ++ normFactors(x) \undocumented

    retractIfCan : ExtP -> Union(P, "failed")
      ++ retractIfCan(x) \undocumented

    Frobenius : ExtP -> ExtP
      ++ Frobenius(x) \undocumented

  CODE ==> add

    normFactors(p:ExtP):List ExtP ==
      facs : List ExtP := [p]
      for i in 1..n-1 repeat 
         member?((p := Frobenius p), facs) => return facs
         facs := cons(p, facs)
      facs

    Frobenius(ff:ExtP):ExtP ==
      fft:ExtP:=0
      while ff^=0 repeat
        fft:=fft + monomial(map(Frobenius, leadingCoefficient ff),
                            degree ff)
        ff:=reductum ff
      fft

    retractIfCan(ff:ExtP):Union(P, "failed") ==          
      fft:P:=0
      while ff ^= 0 repeat
        lc : SUEx := leadingCoefficient ff
        plc: SUP F := 0
        while lc ^= 0 repeat
           lclc:ExtF := leadingCoefficient lc
           (retlc := retractIfCan lclc) case "failed" => return "failed"
           plc := plc + monomial(retlc::F, degree lc)
           lc := reductum lc
        fft:=fft+monomial(plc, degree ff)
        ff:=reductum ff
      fft