source: git/Singular/LIB/sagbi.lib @ e31d29

//////////////////////////////////////////////////////////////////////////////
version="$Id$";
category="Commutative Algebra";
info="
LIBRARY: sagbi.lib  Compute SAGBI basis (subalgebra bases analogous to Groebner bases for ideals) of a subalgebra
AUTHORS: Jan Hackfeld,     Jan.Hackfeld@rwth-aachen.de
         Gerhard Pfister,  pfister@mathematik.uni-kl.de
         Viktor Levandovskyy,     levandov@math.rwth-aachen.de

OVERVIEW:
SAGBI stands for 'subalgebra bases analogous to Groebner bases for ideals'.
It is an important tools in working with finitely presented subalgebras,
many properties of SAGBI bases are analogously important to Groebner bases
of ideals. Note, that due to absence of Noetherian property, SAGBI basis
of a finite number of generators of a subalgebra may be infinite.
Hence we provide procedures, which perform a given number of steps in
computations.

Guide: new implementations of sagbi, sagbiPart, sagbiReduce, sagbiSPoly 
as well as algebraicDependence by Jan Hackfeld do not support computations 
over a quotient ring yet. On the contrary, sagbiReduction and sagbiNF do
work over any ring.

PROCEDURES:
 sagbiSPoly(A [,r,m]); computes SAGBI S-polynomials of A
 sagbiNF(id,dom,k[,n]); computes SAGBI normal form of id wrt dom
 sagbiReduce(I,A [,t,mt]);  performs subalgebra reduction of I by A
 sagbiReduction(I,A [,n]);  performs subalgebra reduction of I by A in a quotient ring
 sagbi(A [,m,t]);      computes SAGBI basis for A
 sagbiPart(A,k[,m]);   computes partial SAGBI basis for A
 algebraicDependence(I,it); performs iterations of SAGBI for algebraic dependencies of I

SEE ALSO: algebra_lib
";


//AUXILIARY PROCEDURES:
//uniqueVariableName(s)                              adds character "@" at the beginning of string s until this is a unique new variable name
//extendRing(r, ...)                                         creates a new ring, which is an extension of r
//stdKernPhi(kernNew, kernOld,...) computes Groebner basis of kernNew+kernOld
//spolynomialsGB(A,...)                                  computes the SAGBI S-polynomials of the subalgebra defined by the generators in A using Groebner bases
//spolynomialsToric(A)                              computes the SAGBI S-polynomials of the subalgebra defined by the generators in A using toric.lib
//reductionGB(ideal F, ideal A,....)         performs subalgebra reduction of F by A
//reduceByMonomials(A)                                        performs subalgebra reduction of all polynomials in A by the subset of monomials

LIB "elim.lib"; //for nselect
LIB "toric.lib"; //for toric_ideal
LIB "algebra.lib";
//////////////////////////////////////////////////////////////////////////////


//static
proc tst_sagbi()
{
  example sagbiSPoly;
  example sagbiNF;
  example sagbiReduce;
  example sagbiReduction;
  example sagbi;
  example sagbiPart;
  example algebraicDependence;
}

static proc assumeQring()
{
  if (ideal(basering) != 0)
  {
    ERROR("This function has not yet been implemented over qrings.");
  }
}


static proc uniqueVariableName (string variableName)
{
  //Adds character "@" at the beginning of variableName until this name ist unique
  //(not contained in the names of the ring variables or description of the coefficient field)
  string ringVars = charstr(basering) + "," + varstr(basering);
  while (find(ringVars,variableName) <> 0)
  {
    variableName="@"+variableName;
  }
  return(variableName);
}

static proc extendRing(r, ideal leadTermsAlgebra, int method) {
  /* Extends ring r with additional variables. If k=ncols(leadTermsAlgebra) and
   * r contains already m additional variables @y, the procedure adds k-m variables
   * @y(m+1)...@y(k) to the ring.
   * The monomial ordering of the extended ring depends on method.
   * Important: When calling this function, the basering (where algebra is defined) has to be active
   */
  def br=basering;
  int i;
  ideal varsBasering=maxideal(1);
  int numTotalAdditionalVars=ncols(leadTermsAlgebra);
  string variableName=uniqueVariableName("@y"); //get a variable name different from existing variables

  //-------- extend current baserring r with new variables @y, one for each new element in ideal algebra  -------------
  list l = ringlist(r);
  for (i=nvars(r)-nvars(br)+1; i<=numTotalAdditionalVars;i++)
  {
    l[2][i+nvars(br)]=string(variableName,"(",i,")");
  }
  if (method>=0 && method<=1)
  {
    if (nvars(r)==nvars(br))
    {        //first run of spolynomialGB in sagbi construction algorithms
      l[3][size(l[3])+1]=l[3][size(l[3])]; //save module ordering
      l[3][size(l[3])-1]=list("dp",intvec(1:numTotalAdditionalVars));
    }
    else
    {        //overwrite existing order for @y(i) to only get one block for the @y
      l[3][size(l[3])-1]=list("dp",intvec(1:numTotalAdditionalVars));
    }
  }
  // VL : todo noncomm case: correctly use l[5] and l[6]
  // that is update matrices
  // at the moment this is troublesome, so use nc_algebra call
  // see how it done in algebraicDependence proc // VL
  def rNew=ring(l);
  setring br;
  return(rNew);
}


static proc stdKernPhi(ideal kernNew, ideal kernOld, ideal leadTermsAlgebra,int method)
{
  /* Computes Groebner basis of kernNew+kernOld, where kernOld already is a Groebner basis
   * and kernNew contains elements of the form @y(i)-leadTermsAlgebra[i] added to it.
   * The techniques chosen is specified by the integer method
   */
  ideal kern;
  attrib(kernOld,"isSB",1);
  if (method==0)
  {
    kernNew=reduce(kernNew,kernOld);
    kern=kernOld+kernNew;
    kern=std(kern);
    //kern=std(kernOld,kernNew); //Found bug using this method. TODO Change if bug is removed
    //this call of std return Groebner Basis of ideal kernNew+kernOld given that kernOld is a Groebner basis
  }
  if (method==1)
  {
    kernNew=reduce(kernNew,kernOld);
    kern=slimgb(kernNew+kernOld);
  }
  return(kern);
}


static proc spolynomialsGB(ideal algebra,r,int method)
{
  /* This procedure does the actual S-polynomial calculation using Groebner basis methods and is
   * called by the procedures sagbiSPoly,sagbi and sagbiPart. As this procedure is called
   * at each step of the SAGBI construction algorithm, we can reuse the information already calculated
   * which is contained in the ring r. This is done in the following order
   * 1. If r already contain m additional variables and m'=number of elements in algebra, extend r with variables @y(m+1),...,@y(m')
   * 2. Transfer all objects to this ring, kernOld=kern is the Groebnerbasis already computed
   * 3. Define ideal kernNew containing elements of the form leadTermsAlgebra(m+1)-@y(m+1),...,leadTermsAlgebra(m')-@y(m')
   * 4. Compute Groebnerbasis of kernOld+kernNew
   * 5. Compute the new algebraic relations
   */
  def br=basering;
  ideal varsBasering=maxideal(1);
  ideal leadTermsAlgebra=lead(algebra);
  //save leading terms as ordering in ring extension may not be compatible with ordering in basering
  int numGenerators=ncols(algebra);

  def rNew=extendRing(r,leadTermsAlgebra,method); // important: br has to be active here
  setring r;
  if (!defined(kern)) //only true for first run of spolynomialGB in sagbi construction algorithms
  {
    ideal kern=0;
    ideal algebraicRelations=0;
  }
  setring rNew;
  //-------------------------- transfer object to new ring rNew ----------------------------------------
  ideal varsBasering=fetch(br,varsBasering);
  ideal kernOld,algebraicRelationsOld;
  kernOld=fetch(r,kern); //kern is Groebner basis of the kernel of the map Phi:r->K[x_1,...,x_n], x(i)->x(i), @y(i)->leadTermsAlgebra(i)
  algebraicRelationsOld=fetch(r,algebraicRelations);
  ideal leadTermsAlgebra=fetch(br,leadTermsAlgebra);
  ideal listOfVariables=maxideal(1);
  //-----------------------define kernNew containing elements to be added to the ideal kern -------------
  ideal kernNew;
  for (int i=nvars(r)-nvars(br)+1; i<=numGenerators; i++)
  {
    kernNew[i-nvars(r)+nvars(br)]=leadTermsAlgebra[i]-listOfVariables[i+nvars(br)];
  }
  //-------------------------- calulate kernel of Phi depending on method choosen -----------------------

  attrib(kernOld,"isSB",1);
  ideal kern=stdKernPhi(kernNew,kernOld,leadTermsAlgebra,method);
  //-------------------------- calulate algebraic relations -----------------------
  ideal algebraicRelations=nselect(kern,1..nvars(br));
  attrib(algebraicRelationsOld,"isSB",1);
  ideal algebraicRelationsNew=reduce(algebraicRelations,algebraicRelationsOld);
  /*        algebraicRelationsOld is a groebner basis by construction (as variable ordering is
   *        block ordering we have an elemination ordering for the varsBasering)
   *        Therefore, to only get the new algebraic relations, calculate
   *        <algebraicRelations>\<algebraicRelationsOld> using groebner reduction
   */
  kill kernOld,kernNew,algebraicRelationsOld,listOfVariables;
  export algebraicRelationsNew,algebraicRelations,kern;
  setring br;
  return(rNew);
}

static proc spolynomialsToric(ideal algebra) {
  /* This procedure does the actual S-polynomial calculation using toric.lib for
   * computation of a Groebner basis for the toric ideal kern(phi), where
   * phi:K[y_1,...,y_m]->K[x_1,...,x_n], y_i->leadmonom(algebra[i])
   * By suitable substitutions we obtain the kernel of the map
   * K[y_1,...,y_m]->K[x_1,...,x_n], x(i)->x(i), @y(i)->leadterm(algebra[i])
   */
  def br=basering;
  int m=ncols(algebra);
  int n=nvars(basering);
  intvec tempVec;
  int i,j;
  ideal leadCoefficients;
  for (i=1;i<=m; i++)
  {
    leadCoefficients[i]=leadcoef(algebra[i]);
  }
  int k=1;
  for (i=1;i<=n;i++)
  {
    for (j=1; j<=m; j++)
    {
      tempVec[k]=leadexp(algebra[j])[i];
      k++;
    }
  }
  //The columns of the matrix A are now the exponent vectors of the leadings monomials in algebra.
  intmat A[n][m]=intmat(tempVec,n,m);
  //Create the preimage ring K[@y(1),...,@y(m)], where m=ncols(algebra).
  string variableName=uniqueVariableName("@y");
  list l = ringlist(basering);
  for (i=1; i<=m;i++)
  {
    l[2][i]=string(variableName,"(",i,")");
  }
  l[3][2]=l[3][size(l[3])];
  l[3][1]=list("dp",intvec(1:m));
  def rNew=ring(l);
  setring rNew;
  //Use toric_ideal to compute the kernel
  ideal algebraicRelations=toric_ideal(A,"ect");
  //Suitable substitution
  ideal leadCoefficients=fetch(br,leadCoefficients);
  for (i=1; i<=m; i++)
  {
    if (leadCoefficients[i]!=0)
    {
      algebraicRelations=subst(algebraicRelations,var(i),1/leadCoefficients[i]*var(i));
    }
  }
  export algebraicRelations;
  return(rNew);
}


static proc reductionGB(ideal F, ideal algebra,r, int tailreduction,int method,int parRed)
{
  /* This procedure does the actual SAGBI/subalgebra reduction using Groebner basis methods and is
   * called by the procedures sagbiReduce,sagbi and sagbiPart
   * If r already is an extension of the basering and contains the ideal kern needed for the subalgebra reduction,
   * the reduction can be started directly, at each reduction step using the fact that
   * p=reduce(leadF,kern) in K[@y(1),...,@y(m)] <=> leadF in K[lead(algebra)]
   * Otherwise some precomputation has to be done, outlined below.
   */
  def br=basering;
  int numVarsBasering=nvars(br);
  ideal varsBasering=maxideal(1);
  int i;

  if (numVarsBasering==nvars(r))
  {
    /* Case that ring r is the same ring as the basering. Using proc extendRing, stdKernPhi
     * one construct the extension of the current baserring with new variables @y, one for each element
     * in ideal algebra and calculates the kernel of Phi, where
     * Phi: r---->br, x_i-->x_i, y_i-->f_i, algebra={f_1,...f_m}, br=K[x1,...,x_n] und r=K[x1,...x_n,@y1,...@y_m]
     * This is similarly done (however step by step for each run of the SAGBI construction algorithm) in the procedure spolynomialsGB
     */
    ideal leadTermsAlgebra=lead(algebra);
    kill r;
    def r=extendRing(br,leadTermsAlgebra,method);
    setring r;
    ideal listOfVariables=maxideal(1);
    ideal leadTermsAlgebra=fetch(br,leadTermsAlgebra);
    ideal kern;
    for (i=1; i<=ncols(leadTermsAlgebra); i++)
    {
      kern[i]=leadTermsAlgebra[i]-listOfVariables[numVarsBasering+i];
    }
    kern=stdKernPhi(kern,0,leadTermsAlgebra,method);
  }
  setring r;
  poly p,leadF;
  ideal varsBasering=fetch(br,varsBasering);
  setring br;
  map phi=r,varsBasering,algebra;
  poly p,normalform,leadF;
  intvec tempExp;
  //------------------algebraic reduction for each polynomial F[i] ---------------------------------------
  for (i=1; i<=ncols(F);i++)
  {
    normalform=0;
    while (F[i]!=0)
    {
      leadF=lead(F[i]);
      if(leadmonom(leadF)==1) { //K is always contained in the subalgebra, thus the remainder is zero in this case
        if (parRed) { break; }
        else { F[i]=0; break; }
      }
      //note: as the ordering in br and r might not be compatible it can be that lead(F[i]) in r is
      //different from lead(F[i]) in br. To take the "correct" leading term therefore take lead(F[i])
      //in br and transfer it to the extension r
      setring r;
      leadF=fetch(br,leadF);
      p=reduce(leadF,kern);
      if (leadmonom(p)<varsBasering[numVarsBasering])
      {        //as choosen ordering is a block ordering, lm(p) in K[y_1...y_m] is equivalent to lm(p)<x_n
        //Needs to be changed, if no block ordering is used!
        setring br;
        F[i]=F[i]-phi(p);
      }
      else
      {
        if (tailreduction)
        {
          setring br;
          normalform=normalform+lead(F[i]);
          F[i]=F[i]-lead(F[i]);
        }
        else
        {
          setring br;
          break;
        }
      }
    }
    if (tailreduction)
    {
      F[i] = normalform;
    }
  }
  return(F);
}

static proc reduceByMonomials(ideal algebra)
/*This procedures uses the sagbiReduce procedure to reduce all polynomials in algebra, which
 * are not monomials, by the subset of all monomials.
 */
{
  ideal monomials;
  int i;
  for (i=1; i<=ncols(algebra);i++)
  {
    if(size(algebra[i])==1)
    {
      monomials[i]=algebra[i];
      algebra[i]=0;
    }
    else
    {
      monomials[i]=0;
    }
  }
  //Monomials now contains the subset of all monomials in algebra, algebra contains the non-monomials.
  if(size(monomials)>0)
  {
    algebra=sagbiReduce(algebra,monomials,1);
    for (i=1; i<=ncols(algebra);i++)
    {
      if(size(monomials[i])==1)
      {
        //Put back monomials into algebra.
        algebra[i]=monomials[i];
      }
    }
  }
  return(algebra);
}


static proc sagbiConstruction(ideal algebra, int iterations, int tailreduction, int method,int parRed)
/* This procedure is the SAGBI construction algorithm and does the actual computation both for
 * the procedure sagbi and sagbiPart.
 * - If the sagbi procedure calls this procedure, iterations==-1 and this procedure only stops
 *   if all S-Polynomials reduce to zero (criterion for termination of SAGBI construction algorithm).
 * - If the sagbiPart procedure calls this procedure, iterations>=0 and iterations specifies the
 *   number of iterations. A degree boundary is not used here.
 * Note that parRed is used for testing a special modification and can be ignored (assume parRed==0).
 */
{
  def br=basering;
  int i=1;

  ideal reducedParameters;
  int numReducedParameters=1;
  int j;
  if (parRed==0)
  {
    algebra=reduceByMonomials(algebra);
    algebra=simplify(simplify(algebra,3),4);
  }
  int step=1;
  if (iterations==-1) //case: infintitly many iterations
  {
    step=0;
    iterations=1;
  }
  ideal P=1; //note: P is initialized this way, so that the while loop is entered. P gets overriden there, anyhow.
  ideal varsBasering=maxideal(1);
  map phi;
  ideal spolynomialsNew;
  def r=br;
  while (size(P)>0 && i<=iterations)
  {
    def rNew=spolynomialsGB(algebra,r,method);
    kill r;
    def r=rNew;
    kill rNew;
    phi=r,varsBasering,algebra;
    spolynomialsNew=simplify(phi(algebraicRelationsNew),6);
    //By construction spolynomialsNew only contains the spolynomials, that have not already
    //been calculated in the steps before.
    P=reductionGB(spolynomialsNew,algebra,r,tailreduction,method,parRed);
    if (parRed)
    {
      for(j=1; j<=ncols(P); j++)
      {
        if (leadmonom(P[j])==1)
        {
          reducedParameters[numReducedParameters]=P[j];
          P[j]=0;
          numReducedParameters++;
        }
      }
    }
    if (parRed==0)
    {
      P=reduceByMonomials(P);  //Reducing with monomials is cheap and can only result in less terms
      P=simplify(simplify(P,3),4); //Avoid that zeros are added to the bases or one element in P more than once
    }
    else
    {
      P=simplify(P,6);
    }
    algebra=algebra,P;         //Note that elements and order of elements must in algebra must not be changed, otherwise the already calculated
    //ideal in r will give wrong results. Thus it is important to use a komma here.
    i=i+step;
  }
  if (step==1)
  { //case that sagbiPart called this procedure
    if (size(P)==0)
    {
      dbprint(4-voice,
              "//SAGBI construction algorithm terminated after "+string(i-1)+" iterations, as all SAGBI S-polynomials reduced to 0.
//Returned generators therefore are a SAGBI basis.");
    }
    else
    {
      dbprint(4-voice,
              "//SAGBI construction algorithm stopped as it reached the limit of "+string(iterations)+" iterations.
//In general the returned generators are no SAGBI basis for the given algebra.");
    }
  }
  kill r;
  if (parRed)
  {
    algebra=algebra,reducedParameters;
  }
  algebra=simplify(algebra,6);
  return(algebra);
}


proc sagbiSPoly(ideal algebra,list #)
"USAGE:   sagbiSPoly(A[, returnRing, meth]);  A is an ideal, returnRing and meth are integers.
RETURN:   ideal or ring
ASSUME: basering is not a qring
PURPOSE: Returns SAGBI S-polynomials of the leading terms of given ideal A if returnRing=0.
@*       Otherwise returns a new ring containing the ideals algebraicRelations
@*       and spolynomials, where these objects are explained by their name.
@*       See the example on how to access these objects.
@format     The other optional argument meth determines which method is
            used for computing the algebraic relations.
            - If meth=0 (default), the procedure std is used.
            - If meth=1, the procedure slimgb is used.
            - If meth=2, the prodecure uses toric_ideal.
@end format
EXAMPLE:  example sagbiSPoly; shows an example"
{
  assumeQring();
  int returnRing;
  int method=0;
  def br=basering;
  ideal spolynomials;
  if (size(#)>=1)
  {
    if (typeof(#[1])=="int")
    {
      returnRing=#[1];
    }
    else
    {
      ERROR("Type of first optional argument needs to be int.");
    }
  }
  if (size(#)==2)
  {
    if (typeof(#[2])=="int")
    {
      if (#[2]<0 || #[2]>2)
      {
        ERROR("Type of second optional argument needs to be 0,1 or 2.");
      }
      else
      {
        method=#[2];
      }
    }
    else
    {
      ERROR("Type of second optional argument needs to be int.");
    }
  }
  if (method>=0 and method<=1)
  {
    ideal varsBasering=maxideal(1);
    def rNew=spolynomialsGB(algebra,br,method);
    map phi=rNew,varsBasering,algebra;
    spolynomials=simplify(phi(algebraicRelationsNew),7);
  }
  if(method==2)
  {
    def r2=spolynomialsToric(algebra);
    map phi=r2,algebra;
    spolynomials=simplify(phi(algebraicRelations),7);
    def rNew=extendRing(br,lead(algebra),0);
    setring rNew;
    ideal algebraicRelations=imap(r2,algebraicRelations);
    export algebraicRelations;
    setring br;
  }

  if (returnRing==0)
  {
    return(spolynomials);
  }
  else
  {
    setring rNew;
    ideal spolynomials=fetch(br,spolynomials);
    export spolynomials;
    setring br;
    return(rNew);
  }
}
example
{ "EXAMPLE:"; echo = 2;
  ring r= 0,(x,y),dp;
  ideal A=x*y+x,x*y^2,y^2+y,x^2+x;
  //------------------ Compute the SAGBI S-polynomials only
  sagbiSPoly(A);
  //------------------ Extended ring is to be returned, which contains
  // the ideal of algebraic relations and the ideal of the S-polynomials
  def rNew=sagbiSPoly(A,1);  setring rNew;
  spolynomials;
  algebraicRelations;
  //----------------- Now we verify that the substitution of A[i] into @y(i)
  // results in the spolynomials listed above
  ideal A=fetch(r,A);
  map phi=rNew,x,y,A;
  ideal spolynomials2=simplify(phi(algebraicRelations),1);
  spolynomials2;
}


proc sagbiReduce(idealORpoly, ideal algebra, list #)
"USAGE: sagbiReduce(I, A[, tr, mt]); I, A ideals, tr, mt optional integers
RETURN: ideal of remainders of I after SAGBI reduction by A
ASSUME: basering is not a qring
PURPOSE:
@format
    The optional argument tr=tailred determines whether tail reduction will be performed.
     - If (tailred=0), no tail reduction is done.
     - If (tailred<>0), tail reduction is done.
     The other optional argument meth determines which method is
         used for Groebner basis computations.
         - If mt=0 (default), the procedure std is used.
         - If mt=1, the procedure slimgb is used.
@end format
EXAMPLE:  example sagbiReduce; shows an example"
{
  assumeQring();
  int tailreduction=0; //Default
  int method=0; //Default
  ideal I;
  if(typeof(idealORpoly)=="ideal")
  {
    I=idealORpoly;
  }
  else
  {
    if(typeof(idealORpoly)=="poly")
    {
      I[1]=idealORpoly;
    }
    else
    {
      ERROR("Type of first argument needs to be an ideal or polynomial.");
    }
  }
  if (size(#)>=1)
  {
    if (typeof(#[1])=="int")
    {
      tailreduction=#[1];
    }
    else
    {
      ERROR("Type of optional argument needs to be int.");
    }
  }
  if (size(#)>=2 )
  {
    if (typeof(#[2])=="int")
    {
      if (#[2]<0 || #[2]>1)
      {
        ERROR("Type of second optional argument needs to be 0 or 1.");
      }
      else
      {
        method=#[2];
      }
    }
    else
    {
      ERROR("Type of optional arguments needs to be int.");
    }
  }

  def r=basering;
  I=simplify(reductionGB(I,algebra,r,tailreduction,method,0),1);

  if(typeof(idealORpoly)=="ideal")
  {
    return(I);
  }
  else
  {
    if(typeof(idealORpoly)=="poly")
    {
      return(I[1]);
    }
  }
}
example
{ "EXAMPLE:"; echo = 2;
  ring r=0,(x,y,z),dp;
  ideal A=x2,2*x2y+y,x3y2;
  poly p1=x^5+x2y+y;
  poly p2=x^16+x^12*y^5+6*x^8*y^4+x^6+y^4+3;
  ideal P=p1,p2;
  //---------------------------------------------
  //SAGBI reduction of polynomial p1 by algebra A. Default call, that is, no tail-reduction is done.
  sagbiReduce(p1,A);
  //---------------------------------------------
  //SAGBI reduction of set of polynomials P by algebra A, now tail-reduction is done.
  sagbiReduce(P,A,1);
}

proc sagbi(ideal algebra, list #)
"USAGE:   sagbi(A[, tr, mt]); A ideal, tr, mt optional integers
RETURN: ideal, a SAGBI basis for A
ASSUME: basering is not a qring
PURPOSE: Computes a SAGBI basis for the subalgebra given by the generators in A.
@format
    The optional argument tr=tailred determines whether tail reduction will be performed.
     - If (tailred=0), no tail reduction is performed,
     - If (tailred<>0), tail reduction is performed.
     The other optional argument meth determines which method is
         used for Groebner basis computations.
         - If mt=0 (default), the procedure std is used.
         - If mt=1, the procedure slimgb is used.
@end format
EXAMPLE:  example sagbi; shows an example"
{
  assumeQring();
  int tailreduction=0; //default value
  int method=0; //default value
  if (size(#)>=1)
  {
    if (typeof(#[1])=="int")
    {
      tailreduction=#[1];
    }
    else
    {
      ERROR("Type of optional argument needs to be int.");
    }
  }
  if (size(#)>=2 )
  {
    if (typeof(#[2])=="int")
    {
      if (#[2]<0 || #[2]>1)
      {
        ERROR("Type of second optional argument needs to be 0 or 1.");
      }
      else
      {
        method=#[2];
      }
    }
    else
    {
      ERROR("Type of optional arguments needs to be int.");
    }
  }
  ideal a;
  a=sagbiConstruction(algebra,-1,tailreduction,method,0);
  a=simplify(a,7);
  a=interreduced(a);
  return(a);
}
example
{ "EXAMPLE:"; echo = 2;
  ring r= 0,(x,y,z),dp;
  ideal A=x2,y2,xy+y;
  //Default call, no tail-reduction is done.
  sagbi(A);
  //---------------------------------------------
  //Call with tail-reduction and method specified.
  sagbi(A,1,0);
}

proc sagbiPart(ideal algebra, int iterations, list #)
"USAGE: sagbiPart(A, k,[tr, mt]); A is an ideal, k, tr and mt are integers
RETURN: ideal
ASSUME: basering is not a qring
PURPOSE: Performs k iterations of the SAGBI construction algorithm for the subalgebra given by the generators given by A.
@format
     The optional argument tr=tailred determines if tail reduction will be performed.
     - If (tailred=0), no tail reduction is performed,
     - If (tailred<>0), tail reduction is performed.
     The other optional argument meth determines which method is
         used for Groebner basis computations.
         - If mt=0 (default), the procedure std is used.
         - If mt=1, the procedure slimgb is used.
@end format
EXAMPLE:  example sagbiPart; shows an example"
{
  assumeQring();
  int tailreduction=0; //default value
  int method=0; //default value
  if (size(#)>=1)
  {
    if (typeof(#[1])=="int")
    {
      tailreduction=#[1];
    }
    else
    {
      ERROR("Type of optional argument needs to be int.");
    }
  }
  if (size(#)>=2 )
  {
    if (typeof(#[2])=="int")
    {
      if (#[2]<0 || #[2]>3)
      {
        ERROR("Type of second optional argument needs to be 0 or 1.");
      }
      else
      {
        method=#[2];
      }
    }
    else
    {
      ERROR("Type of optional arguments needs to be int.");
    }
  }
  if (iterations<0)
  {
    ERROR("Number of iterations needs to be non-negative.");
  }
  ideal a;
  a=sagbiConstruction(algebra,iterations,tailreduction,method,0);
  a=simplify(a,7);
  a=interreduced(a);
  return(a);
}
example
{ "EXAMPLE:"; echo = 2;
  ring r= 0,(x,y,z),dp;
  //The following algebra does not have a finite SAGBI basis.
  ideal A=x,xy-y2,xy2;
  //---------------------------------------------------
  //Call with two iterations, no tail-reduction is done.
  sagbiPart(A,2);
  //---------------------------------------------------
  //Call with three iterations, tail-reduction and method 0.
  sagbiPart(A,3,1,0);
}



// VL: finished the documentation
// TO compare with algDependent from algebra_lib
// TODO : remove constants from algebraic dependencies
proc algebraicDependence(ideal I,int iterations)
"USAGE: algebraicDependence(I,it); I an an ideal, it is an integer
RETURN: ring
ASSUME: basering is not a qring
PURPOSE: In @code{it} iterations, compute algebraic dependencies between elements of I
EXAMPLE: example algebraicDependence; shows an example"
{
  assumeQring();
  def br=basering;
  int i;

  string parameterName=uniqueVariableName("@c");
  list l = ringlist(basering);
  list parList;
  for (i=1; i<=ncols(I);i++)
  {
    parList[i]=string(parameterName,"(",i,")");
  }
  l[1]=list(l[1],parList,list(list("dp",1:ncols(I))));
  ideal temp=0;
  l[1][4]=temp;
  // addition VL: noncomm case
  int isNCcase = 0; // default for comm
  //         if (size(l)>4)
  //         {
  //           // that is we're in the noncomm algebra
  //           isNCcase = 1; // noncomm
  //           matrix @C@ = l[5];
  //           matrix @D@ = l[6];
  //           l = l[1],l[2],l[3],l[4];
  //         }
  def parameterRing=ring(l);

  string extendVarName=uniqueVariableName("@c");
  list l2 = ringlist(basering);
  for (i=1; i<=ncols(I);i++)
  {
    l2[2][i+nvars(br)]=string(extendVarName,"(",i,")");
  }
  l2[3][size(l2[3])+1]=l2[3][size(l2[3])];
  l2[3][size(l2[3])-1]=list("dp",intvec(1:ncols(I)));
  //         if (isNCcase)
  //         {
  //           // that is we're in the noncomm algebra
  //           matrix @C@2 = l2[5];
  //           matrix @D@2 = l2[6];
  //           l2 = l2[1],l2[2],l2[3],l2[4];
  //         }

  def extendVarRing=ring(l2);
  setring extendVarRing;
  // VL : this requires extended matrices
  // let's forget it for the moment
  // since this holds only for showing the answer
  //         if (isNCcase)
  //         {
  //           matrix C2=imap(br,@C@2);
  //           matrix D2=imap(br,@D@2);
  //           def er2 = nc_algebra(C2,D2);
  //           setring er2;
  //           def extendVarRing=er2;
  //         }

  setring parameterRing;

  //         if (isNCcase)
  //         {
  //           matrix C=imap(br,@C@);
  //           matrix D=imap(br,@D@);
  //           def pr = nc_algebra(C,D);
  //           setring pr;
  //           def parameterRing=pr;
  //         }


  ideal I=fetch(br,I);
  ideal algebra;
  for (i=1; i<=ncols(I);i++)
  {
    algebra[i]=I[i]-par(i);
  }
  algebra=sagbiConstruction(algebra, iterations,0,0,1);
  int j=1;
  ideal algDep;
  for (i=1; i<= ncols(algebra); i++)
  {
    if (leadmonom(algebra[i])==1)
    {
      algDep[j]=algebra[i];
      j++;
    }
  }
  setring extendVarRing;
  ideal algDep=imap(parameterRing,algDep);
  ideal algebra=imap(parameterRing,algebra);
  export algDep,algebra;
  //print(algDep);
  setring br;
  return(extendVarRing);
}
example
{ "EXAMPLE:"; echo = 2;
  ring r= 0,(x,y),dp;
  //The following algebra does not have a finite SAGBI basis.
  ideal I=x^2, xy-y2, xy2;
  //---------------------------------------------------
  //Call with two iterations
  def DI = algebraicDependence(I,2);
  setring DI; algDep;
  // we see that no dependency has been seen so far
  //---------------------------------------------------
  //Call with two iterations
  setring r; kill DI;
  def DI = algebraicDependence(I,3);
  setring DI; algDep;
  map F = DI,x,y,x^2, xy-y2, xy2;
  F(algDep); // we see that it is a dependence indeed
}

static proc interreduced(ideal I)
{
  ideal J,B;
  int i,j,k;
  poly f;
  for(k=1;k<=ncols(I);k++)
  {
    f=I[k];
    I[k]=0;
    f=sagbiReduce(f,I,1);
    I[k]=f;
  }
  I=simplify(I,2);
  return(I);
}
///////////////////////////////////////////////////////////////////////////////

proc sagbiReduction(poly p,ideal dom,list #)
"USAGE: sagbiReduction(p,dom[,n]); p poly , dom  ideal
RETURN: polynomial, after one step of subalgebra reduction
PURPOSE:
@format
    Three algorithm variants are used to perform subalgebra reduction.
    The positive interger n determines which variant should be used.
    n may take the values 0 (default), 1 or 2.
@end format
NOTE: works over both polynomial rings and their quotients
EXAMPLE: example sagbiReduction; shows an example"
{
  def bsr=basering;
  ideal B=ideal(bsr);//When the basering is quotient ring  this type casting
                     // gives the quotient ideal.
  int b=size(B);
  int n=nvars(bsr);

  //In quotient rings, SINGULAR, usually does not reduce polynomials w.r.t the
  //quotient ideal,therefore we should first  reduce ,when it is necessary for computations,
  // to have a uniquely determined representant for each equivalent
  //class,which is the case of this algorithm.

  if(b !=0) //means that the basering is a quotient ring
  {
    p=reduce(p,std(0));
    dom=reduce(dom,std(0));
  }

  int i,choose;
  int z=ncols(dom);

  if((size(#)>0) && (typeof(#[1])=="int"))
  {
    choose = #[1];
  }
  if (size(#)>1)
  {
    choose =#[2];
  }

  //=======================first algorithm(default)=========================
  if ( choose == 0 )
  {
    list L = algebra_containment(lead(p),lead(dom),1);
    if( L[1]==1 )
    {
      // the ring L[2] = char(bsr),(x(1..nvars(bsr)),y(1..z)),(dp(n),dp(m)),
      // contains poly check s.t. LT(p) is of the form check(LT(f1),...,LT(fr))
      def s1 = L[2];
      map psi = s1,maxideal(1),dom;
      poly re = p - psi(check);
      // divide by the maximal power of #[1]
      if ( (size(#)>0) && (typeof(#[1])=="poly") )
      {
        while ((re!=0) && (re!=#[1]) &&(subst(re,#[1],0)==0))
        {
          re=re/#[1];
        }
      }
      return(re);
    }
    return(p);
  }
  //======================2end variant of algorithm=========================
  //It uses two different commands for elimaination.
  //if(choose==1):"elimainate"command.
  //if (choose==2):"nselect" command.
  else
  {
    poly v=product(maxideal(1));

    //------------- change the basering bsr to bsr[@(0),...,@(z)] ----------
    execute("ring s=("+charstr(basering)+"),("+varstr(basering)+",@(0..z)),dp;");
    // Ev hier die Reihenfolge der Vars aendern. Dazu muss unten aber entsprechend
    // geaendert werden:
    //  execute("ring s="+charstr(basering)+",(@(0..z),"+varstr(basering)+"),dp;");

    //constructs the leading ideal of dom=(p-@(0),dom[1]-@(1),...,dom[z]-@(z))
    ideal dom=imap(bsr,dom);
    for (i=1;i<=z;i++)
    {
      dom[i]=lead(dom[i])-var(nvars(bsr)+i+1);
    }
    dom=lead(imap(bsr,p))-@(0),dom;

    //---------- eliminate the variables of the basering bsr --------------
    //i.e. computes dom intersected with K[@(0),...,@(z)].

    if(choose==1)
    {
      ideal kern=eliminate(dom,imap(bsr,v));//eliminate does not need a
                                            //standard basis as input.
    }
    if(choose==2)
    {
      ideal kern= nselect(groebner(dom),1..n);//"nselect" is combinatorial command
                                         //which uses the internal command
                                         // "simplify"
    }

    //---------  test wether @(0)-h(@(1),...,@(z)) is in ker ---------------
    // for some poly h and divide by maximal power of q=#[1]
    poly h;
    z=size(kern);
    for (i=1;i<=z;i++)
    {
      h=kern[i]/@(0);
      if (deg(h)==0)
      {
        h=(1/h)*kern[i];
        // define the map psi : s ---> bsr defined by @(i) ---> p,dom[i]
        setring bsr;
        map psi=s,maxideal(1),p,dom;
        poly re=psi(h);
        // divide by the maximal power of #[1]
        if ((size(#)>0) && (typeof(#[1])== "poly") )
        {
          while ((re!=0) && (re!=#[1]) &&(subst(re,#[1],0)==0))
          {
            re=re/#[1];
          }
        }
        return(re);
      }
    }
    setring bsr;
    return(p);
  }
}
example
{"EXAMPLE:"; echo = 2;
  ring r= 0,(x,y),dp;
  ideal dom =x2,y2,xy-y;
  poly p=x4+x3y+xy2-y2;
  sagbiReduction(p,dom);
  sagbiReduction(p,dom,2);
  // now let us see the action over quotient ring
  ideal I = xy;
  qring Q = std(I);
  ideal dom = imap(r,dom); poly p = imap(r,p);
  sagbiReduction(p,dom,1);
}

proc sagbiNF(id,ideal dom,int k,list#)
"USAGE: sagbiNF(id,dom,k[,n]); id either poly or ideal,dom ideal, k and n positive intergers.
RETURN: same as type of id; ideal or polynomial.
PURPOSE:
@format
    The integer k determines what kind of s-reduction is performed:
    - if (k=0) no tail s-reduction is performed.
    - if (k=1) tail s-reduction is performed.
    Three Algorithm variants are used to perform subalgebra reduction.
    The positive integer n determines which variant should be used.
    n may take the values (0 or default),1 or 2.
@end format
NOTE: sagbiNF works over both rings and quotient rings
EXAMPLE: example sagbiNF; show example "
{
  ideal rs;
  if (ideal(basering) == 0)
  {
    rs = sagbiReduce(id,dom,k) ;
  }
  else
  {
    rs = sagbiReduction(id,dom,k) ;
  }
  return(rs);
}
example
{"EXAMPLE:"; echo = 2;
 ring r=0,(x,y),dp;
 poly p=x4+x2y+y;
 ideal dom =x2,x2y+y,x3y2;
 sagbiNF(p,dom,1);
 ideal I= x2-xy;
 qring Q=std(I); // we go to the quotient ring
 poly p=imap(r,p);
 NF(p,std(0)); // the representative of p has changed
 ideal dom = imap(r,dom);
 print(matrix(NF(dom,std(0)))); // dom has changed as well
 sagbiNF(p,dom,0); // no tail reduction
 sagbiNF(p,dom,1);// tail subalgebra reduction is performed
}

/*
  ring r= 0,(x,y),dp;
  //The following algebra does not have a finite SAGBI basis.
  ideal J=x^2, xy-y2, xy2, x^2*(x*y-y^2)^2 - (x*y^2)^2*x^4 + 11;
  //---------------------------------------------------
  //Call with two iterations
  def DI = algebraicDependence(J,2);
  setring DI; algDep;
*/