source: git/Singular/LIB/freegb.lib @ 285d21

//////////////////////////////////////////////////////////////////////////////
version="$Id: freegb.lib,v 1.7 2008-02-26 23:36:12 levandov Exp $";
category="Noncommutative";
info="
LIBRARY: ratgb.lib  Twosided Noncommutative Groebner bases in Free Algebras
AUTHOR: Viktor Levandovskyy,     levandov@math.rwth-aachen.de

PROCEDURES:
freegbasis(list L, int n);   compute two-sided Groebner basis of ideal, encoded via L, up to degree n
lst2str(list L);         convert a list (of modules) into polynomials in free algebra
mod2str(module M);       convert a module into a polynomial in free algebra
"

// this library computes two-sided GB of an ideal
// in a free associative algebra

// a monomial is encoded via a vector V
// where V[1] = coefficient
// V[1+i] = the corresponding symbol

LIB "qhmoduli.lib"; // for Max

proc vct2mono(vector v)
{
  // produces a monomial in new vars
  // need a ring with new vars!!
}

proc lshift(module M, int s, string varing, def lpring)
{
  // FINALLY IMPLEMENTED AS A PART OT THE CODE
  // shifts a poly from the ring @R to s positions
  // M lives in varing, the result in lpring
  // to be run from varing
  int i, j, k, sm, sv;
  vector v;
  //  execute("setring "+lpring);
  setring lpring;
  poly @@p;
  ideal I;
  execute("setring "+varing);
  sm = ncols(M);
  for (i=1; i<=s; i++)
  {
    // modules, e.g. free polynomials
    for (j=1; j<=sm; j++)
    {
      //vectors, e.g. free monomials
      v  = M[j];
      sv = size(v);
      sp = "@@p = @@p + ";
      for (k=2; k<=sv; k++)
      {
        sp = sp + string(v[k])+"("+string(k-1+s)+")*";
      }
      sp = sp + string(v[1])+";"; // coef;
      setring lpring;
      //      execute("setring "+lpring);
      execute(sp);
      execute("setring "+varing);
    }
    setring lpring;
    //    execute("setring "+lpring);
    I = I,@@p;
    @@p = 0;
  }
  setring lpring;
  //execute("setring "+lpring);
  export(I);
  //  setring varing;
  execute("setring "+varing);
}

proc skip0(vector v)
{
  // skips zeros in vector
  int sv = nrows(v);
  int sw = size(v);
  if (sv == sw)
  {
    return(v);
  }
  int i;
  int j=1;
  vector w;
  for (i=1; i<=sv; i++)
  {
    if (v[i] != 0)
    {
      w = w + v[i]*gen(j);
      j++;
    }
  }
  return(w);
}

proc lst2str(list L)
"USAGE:  lst2str(L);  L a list of modules
RETURN:  list (of strings)
PURPOSE: convert a list (of modules) into polynomials in free algebra
EXAMPLE: example lst2str; shows examples
"
{
  // returns a list of strings
  // being sentences in words built from L
  int i;
  int s    = size(L);
  list N;
  for(i=1; i<=s; i++)
  {
    if ((typeof(L[i]) == "module") || (typeof(L[i]) == "matrix") )
    {
      N[i] = mod2str(L[i]);
    }
    else
    {
      "module or matrix expected in the list";
      return(N);
    }
  }
  return(N);
}
example
{
  "EXAMPLE:"; echo = 2;
  ring r = 0,(x,y,z),(dp(1),dp(2));
  module M = [-1,x,y],[-7,y,y],[3,x,x];
  module N = [1,x,y,x,y],[-2,y,x,y,x],[6,x,y,y,x,y];
  list L; L[1] = M; L[2] = N;
  lst2str(L);
}


proc mod2str(module M)
"USAGE:  mod2str(M);  M a module
RETURN:  string
PURPOSE: convert a modules into a polynomial in free algebra
EXAMPLE: example mod2str; shows examples
"
{
  // returns a string
  // a sentence in words built from M
  int i;
  int s    = ncols(M);
  string t;
  string mp;
  for(i=1; i<=s; i++)
  {
    mp = vct2str(M[i]);
    if (mp[1] == "-")
    {
      t = t + mp;
    }
    else
    {
      t = t + "+" + mp;
    }
  }
  if (t[1]=="+")
  {
    t = t[2..size(t)]; // remove first "+"
  }
  return(t);
}
example
{
  "EXAMPLE:"; echo = 2;
  ring r = 0,(x,y,z),(dp);
  module M = [1,x,y,x,y],[-2,y,x,y,x],[6,x,y,y,x,y];
  mod2str(M);
}

proc vct2str(vector v)
{
  int ppl = printlevel-voice+2;
  // for a word, encoded by v
  // produces a string for it
  v = skip0(v);
  number cf = leadcoef(v[1]);
  int s = size(v);
  string vs,vv,vp,err;
  int i,j,p,q;
  for (i=1; i<=s-1; i++)
  {
    p     = IsVar(v[i+1]);
    if (p==0)
    {
      err = "Error: monomial expected at" + string(i+1);
      dbprint(ppl,err);
      return("_");
    }
    if (p==1)
    {
      vs = vs + string(v[i+1]);
    }
    else //power
    {
      vv = string(v[i+1]);
      q = find(vv,"^");
      if (q==0)
      {
        q = find(vv,string(p));
        if (q==0)
        {
          err = "error in find for string "+vv;
          dbprint(ppl,err);
          return("_");
        }
      }
      // q>0
      vp = vv[1..q-1];
      for(j=1;j<=p;j++)
      {
        vs = vs + vp;
      }
    }
  }
  string scf;
  if (cf == -1)
  {
    scf = "-";
  }
  else
  {
    scf = string(cf);
    if (cf == 1)
    {
      scf = "";
    }
  }
  vs = scf + vs;
  return(vs);
}
example
{
  ring r = (0,a),(x,y3,z(1)),dp;
  vector v = [-7,x,y3^4,x2,z(1)^3];
  vct2str(v);
  vector w = [-7a^5+6a,x,y3,y3,x,z(1),z(1)];
  vct2str(w);
}

proc IsVar(poly p)
{
  // checks whether p is a variable indeed
  // if it's a power of a variable, returns the power
  if (p==0) {  return(0); } //"p=0";
  poly q   = leadmonom(p);
  if ( (p-lead(p)) !=0 ) { return(0); } // "p-lm(p)>0";
  intvec v = leadexp(p);
  int s = size(v);
  int i=1;
  int cnt = 0;
  int pwr = 0;
  for (i=1; i<=s; i++)
  {
    if (v[i] != 0)
    {
      cnt++;
      pwr = v[i];
    }
  }
  //  "cnt:";  cnt;
  if (cnt==1) { return(pwr); }
  else { return(0); }
}
example
{
  ring r = 0,(x,y),dp;
  poly f = xy+1;
  IsVar(f);
  poly g = xy;
  IsVar(g);
  poly h = y^3;
  IsVar(h);
  poly i = 1;
  IsVar(i);
}

// new conversion routines

proc id2words(ideal I, int d)
{
  // input: ideal I of polys in letter-place notation
  // in the ring with d real vars
  // output: the list of strings: associative words
  // extract names of vars
  int i,m,n; string s; string place = "(1)";
  list lv;
  for(i=1; i<=d; i++)
  {
    s = string(var(i));
    // get rid of place
    n = find(s, place);
    if (n>0)
    {
      s = s[1..n-1];
    }
    lv[i] = s;
  }
  poly p,q;
  for (i=1; i<=ncols(I); i++)
  {
    if (I[i] != 0)
    {
      p = I[i];
      while (p!=0)
      {
         q = leadmonom(p);
         
      }
    }
  }

  return(lv);
}
example
{
  "EXAMPLE:"; echo = 2;
  ring r = 0,(x(1),y(1),z(1)),dp;
  ideal I = x(1)*y(2) -z(1)*x(2);
  id2words(I,3);
}



proc mono2word(poly p, int d)
{
  
}

// given the element -7xy^2x, it is represented as [-7,x,y^2,x] or as [-7,x,y,y,x]
// use the orig ord on (x,y,z) and expand it blockwise to (x(i),y(i),z(i))

// the correspondences:
// monomial in K<x,y,z>    <<--->> vector in R
// polynomial in K<x,y,z>  <<--->> list of vectors (matrix/module) in R
// ideal in K<x,y,z>       <<--->> list of matrices/modules in R


// 1. form a new ring
// 2. NOP
// 3. compute GB -> with the kernel stuff
// 4. skip shifted elts (check that no such exist?)
// 5. go back to orig vars, produce strings/modules
// 6. return the result

proc freegbasis(list LM, int d)
"USAGE:  freegbasis(L, d);  L a list of modules, d an integer
RETURN:  ring
PURPOSE: compute the two-sided Groebner basis of an ideal, encoded by L in
the free associative algebra, up to degree d
EXAMPLE: example freegbasis; shows examples
"
{
  // d = up to degree, will be shifted to d+1
  if (d<1) {"bad d"; return(0);}

  int ppl = printlevel-voice+2;
  string err = "";

  int i,j,s;
  def save = basering;
  // determine max no of places in the input
  int slm = size(LM); // numbers of polys in the ideal
  int sm;
  intvec iv;
  module M;
  for (i=1; i<=slm; i++)
  {
    // modules, e.g. free polynomials
    M  = LM[i];
    sm = ncols(M);
    for (j=1; j<=sm; j++)
    {
      //vectors, e.g. free monomials
      iv = iv, size(M[j])-1; // 1 place is reserved by the coeff
    }
  }
  int D = Max(iv); // max size of input words
  if (d<D) {"bad d"; return(LM);}
  D = D + d-1;
  //  D = d;
  list LR  = ringlist(save);
  list L, tmp;
  L[1] = LR[1]; // ground field
  L[4] = LR[4]; // quotient ideal
  tmp  = LR[2]; // varnames
  s = size(LR[2]);
  for (i=1; i<=D; i++)
  {
    for (j=1; j<=s; j++)
    {
      tmp[i*s+j] = string(tmp[j])+"("+string(i+1)+")";
    }
  }
  for (i=1; i<=s; i++)
  {
    tmp[i] = string(tmp[i])+"("+string(1)+")";
  }
  L[2] = tmp;
  list OrigNames = LR[2];
  // ordering: d blocks of the ord on r
  // try to get whether the ord on r is blockord itself
  s = size(LR[3]);
  if (s==2)
  {
    // not a blockord, 1 block + module ord
    tmp = LR[3][s]; // module ord
    for (i=1; i<=D; i++)
    {
      LR[3][s-1+i] = LR[3][1];
    }
    LR[3][s+D] = tmp;
  }
  if (s>2)
  {
    // there are s-1 blocks
    int nb = s-1;
    tmp = LR[3][s]; // module ord
    for (i=1; i<=D; i++)
    {
      for (j=1; j<=nb; j++)
      {
        LR[3][i*nb+j] = LR[3][j];
      }
    }
    //    size(LR[3]);
    LR[3][nb*(D+1)+1] = tmp;
  }
  L[3] = LR[3];
  def @R = ring(L);
  setring @R;
  ideal I;
  poly @p;
  s = size(OrigNames);
  //  "s:";s;
  // convert LM to canonical vectors (no powers)
  setring save;
  kill M; // M was defined earlier
  module M;
  slm = size(LM); // numbers of polys in the ideal
  int sv,k,l;
  vector v;
  //  poly p;
  string sp;
  setring @R;
  poly @@p=0;
  setring save;
  for (l=1; l<=slm; l++)
  {
    // modules, e.g. free polynomials
    M  = LM[l];
    sm = ncols(M); // in intvec iv the sizes are stored
    // modules, e.g. free polynomials
    for (j=1; j<=sm; j++)
    {
      //vectors, e.g. free monomials
      v  = M[j];
      sv = size(v);
      //        "sv:";sv;
      sp = "@@p = @@p + ";
      for (k=2; k<=sv; k++)
      {
        sp = sp + string(v[k])+"("+string(k-1)+")*";
      }
      sp = sp + string(v[1])+";"; // coef;
      setring @R;
      execute(sp);
      setring save;
    }
    setring @R;
    //      "@@p:"; @@p;
    I = I,@@p;
    @@p = 0;
    setring save;
  }
  kill sp;
  // 3. compute GB
  setring @R;
  dbprint(ppl,"computing GB");
  ideal J = system("freegb",I,d,nvars(save));
  //  ideal J = slimgb(I); 
  dbprint(ppl,J);
  // 4. skip shifted elts
  ideal K = select1(J,1,s); // s = size(OrigNames)
  dbprint(ppl,K);
  dbprint(ppl, "done with GB");
  // K contains vars x(1),...z(1) = images of originals
  // 5. go back to orig vars, produce strings/modules
  if (K[1] == 0)
  {
    "no reasonable output, GB gives 0";
    return(0);
  }
  int sk = size(K);
  int sp, sx, a, b;
  intvec x;
  poly p,q;
  poly pn;
  // vars in 'save'
  setring save;
  module N;
  list LN;
  vector V;
  poly pn;
  // test and skip exponents >=2
  setring @R;
  for(i=1; i<=sk; i++)
  {
    p  = K[i];
    while (p!=0)
    {
      q  = lead(p);
      //      "processing q:";q;
      x  = leadexp(q);
      sx = size(x);
      for(k=1; k<=sx; k++)
      {
        if ( x[k] >= 2 )
        {
          err = "skip: the value x[k] is " + string(x[k]);
          dbprint(ppl,err);
          //        return(0);
          K[i] = 0;
          p    = 0;
          q    = 0;
          break;
        }
      }
      p  = p - q;
    }
  }
  K  = simplify(K,2);
  sk = size(K);
  for(i=1; i<=sk; i++)
  {
    //    setring save;
    //    V  = 0;
    setring @R;
    p  = K[i];
    while (p!=0)
    {
      q  = lead(p);
      err =  "processing q:" + string(q);
      dbprint(ppl,err);
      x  = leadexp(q);
      sx = size(x);
      pn = leadcoef(q);
      setring save;
      pn = imap(@R,pn);
      V  = V + leadcoef(pn)*gen(1);
      for(k=1; k<=sx; k++)
      {
        if (x[k] ==1)
        {
          a = k / s; // block number=a+1, a!=0
          b = k % s; // remainder
          //      printf("a: %s, b: %s",a,b);
          if (b == 0)
          {
            // that is it's the last var in the block
            b = s;
            a = a-1;
          }
          V = V + var(b)*gen(a+2);
        }
//      else
//      {
//        printf("error: the value x[k] is %s", x[k]);
//        return(0);
//      }
      }
      err = "V: " + string(V);
      dbprint(ppl,err);
      //      printf("V: %s", string(V));
      N = N,V;
      V  = 0;
      setring @R;
      p  = p - q;
      pn = 0;
    }
    setring save;
    LN[i] = simplify(N,2);
    N     = 0;
  }
  setring save;
  return(LN);
}
example
{
  "EXAMPLE:"; echo = 2;
  ring r = 0,(x,y,z),(dp(1),dp(2));
  module M = [-1,x,y],[-7,y,y],[3,x,x];
  module N = [1,x,y,x],[-1,y,x,y];
  list L; L[1] = M; L[2] = N;
  lst2str(L);
  def U = freegbasis(L,5);
  lst2str(U);
}

proc crs(list LM, int d)
"USAGE:  crs(L, d);  L a list of modules, d an integer
RETURN:  ring
PURPOSE: create a ring and shift the ideal
EXAMPLE: example crs; shows examples
"
{
  // d = up to degree, will be shifted to d+1
  if (d<1) {"bad d"; return(0);}

  int ppl = printlevel-voice+2;
  string err = "";

  int i,j,s;
  def save = basering;
  // determine max no of places in the input
  int slm = size(LM); // numbers of polys in the ideal
  int sm;
  intvec iv;
  module M;
  for (i=1; i<=slm; i++)
  {
    // modules, e.g. free polynomials
    M  = LM[i];
    sm = ncols(M);
    for (j=1; j<=sm; j++)
    {
      //vectors, e.g. free monomials
      iv = iv, size(M[j])-1; // 1 place is reserved by the coeff
    }
  }
  int D = Max(iv); // max size of input words
  if (d<D) {"bad d"; return(LM);}
  D = D + d-1;
  //  D = d;
  list LR  = ringlist(save);
  list L, tmp;
  L[1] = LR[1]; // ground field
  L[4] = LR[4]; // quotient ideal
  tmp  = LR[2]; // varnames
  s = size(LR[2]);
  for (i=1; i<=D; i++)
  {
    for (j=1; j<=s; j++)
    {
      tmp[i*s+j] = string(tmp[j])+"("+string(i)+")";
    }
  }
  for (i=1; i<=s; i++)
  {
    tmp[i] = string(tmp[i])+"("+string(0)+")";
  }
  L[2] = tmp;
  list OrigNames = LR[2];
  // ordering: d blocks of the ord on r
  // try to get whether the ord on r is blockord itself
  s = size(LR[3]);
  if (s==2)
  {
    // not a blockord, 1 block + module ord
    tmp = LR[3][s]; // module ord
    for (i=1; i<=D; i++)
    {
      LR[3][s-1+i] = LR[3][1];
    }
    LR[3][s+D] = tmp;
  }
  if (s>2)
  {
    // there are s-1 blocks
    int nb = s-1;
    tmp = LR[3][s]; // module ord
    for (i=1; i<=D; i++)
    {
      for (j=1; j<=nb; j++)
      {
        LR[3][i*nb+j] = LR[3][j];
      }
    }
    //    size(LR[3]);
    LR[3][nb*(D+1)+1] = tmp;
  }
  L[3] = LR[3];
  def @R = ring(L);
  setring @R;
  ideal I;
  poly @p;
  s = size(OrigNames);
  //  "s:";s;
  // convert LM to canonical vectors (no powers)
  setring save;
  kill M; // M was defined earlier
  module M;
  slm = size(LM); // numbers of polys in the ideal
  int sv,k,l;
  vector v;
  //  poly p;
  string sp;
  setring @R;
  poly @@p=0;
  setring save;
  for (l=1; l<=slm; l++)
  {
    // modules, e.g. free polynomials
    M  = LM[l];
    sm = ncols(M); // in intvec iv the sizes are stored
    for (i=0; i<=d-iv[l]; i++)
    {
      // modules, e.g. free polynomials
      for (j=1; j<=sm; j++)
      {
        //vectors, e.g. free monomials
        v  = M[j];
        sv = size(v);
        //      "sv:";sv;
        sp = "@@p = @@p + ";
        for (k=2; k<=sv; k++)
        {
          sp = sp + string(v[k])+"("+string(k-2+i)+")*";
        }
        sp = sp + string(v[1])+";"; // coef;
        setring @R;
        execute(sp);
        setring save;
      }
      setring @R;
      //      "@@p:"; @@p;
      I = I,@@p;
      @@p = 0;
      setring save;
    }
  }
  setring @R;
  export I;
  return(@R);
}
example
{
  "EXAMPLE:"; echo = 2;
  ring r = 0,(x,y,z),(dp(1),dp(2));
  module M = [-1,x,y],[-7,y,y],[3,x,x];
  module N = [1,x,y,x],[-1,y,x,y];
  list L; L[1] = M; L[2] = N;
  lst2str(L);
  def U = crs(L,5);
  setring U; U;
  I;
}

proc polylen(ideal I)
{
  // returns the ideal of length of polys
  int i;
  intvec J;
  number s = 0;
  for(i=1;i<=ncols(I);i++)
  {
    J[i] = size(I[i]);
    s = s + J[i];
  }
  printf("the sum of length %s",s);
  //  print(s);
  return(J);
}

proc freegbRing(int d)
"USAGE:  freegbRing(d); d an integer
RETURN:  ring
PURPOSE: creates a ring with d blocks of shifted original variables
EXAMPLE: example freegbRing; shows examples
"
{
  // d = up to degree, will be shifted to d+1
  if (d<1) {"bad d"; return(0);}

  int ppl = printlevel-voice+2;
  string err = "";

  int i,j,s;
  def save = basering;
  int D = d-1;
  list LR  = ringlist(save);
  list L, tmp;
  L[1] = LR[1]; // ground field
  L[4] = LR[4]; // quotient ideal
  tmp  = LR[2]; // varnames
  s = size(LR[2]);
  for (i=1; i<=D; i++)
  {
    for (j=1; j<=s; j++)
    {
      tmp[i*s+j] = string(tmp[j])+"("+string(i+1)+")";
    }
  }
  for (i=1; i<=s; i++)
  {
    tmp[i] = string(tmp[i])+"("+string(1)+")";
  }
  L[2] = tmp;
  list OrigNames = LR[2];
  // ordering: d blocks of the ord on r
  // try to get whether the ord on r is blockord itself
  // TODO: make L(2) ordering! exponent is maximally 2
  s = size(LR[3]);
  if (s==2)
  {
    // not a blockord, 1 block + module ord
    tmp = LR[3][s]; // module ord
    for (i=1; i<=D; i++)
    {
      LR[3][s-1+i] = LR[3][1];
    }
    LR[3][s+D] = tmp;
  }
  if (s>2)
  {
    // there are s-1 blocks
    int nb = s-1;
    tmp = LR[3][s]; // module ord
    for (i=1; i<=D; i++)
    {
      for (j=1; j<=nb; j++)
      {
        LR[3][i*nb+j] = LR[3][j];
      }
    }
    //    size(LR[3]);
    LR[3][nb*(D+1)+1] = tmp;
  }
  L[3] = LR[3];
  def @R = ring(L);
  //  setring @R;
  return (@R);
}
example
{
  "EXAMPLE:"; echo = 2;
  ring r = 0,(x,y,z),(dp(1),dp(2));
  def A = freegbRing(2);
  setring A;
  A;
}


proc ex_shift()
{
  LIB "freegb.lib";
  ring r = 0,(x,y,z),(dp(1),dp(2));
  module M = [-1,x,y],[-7,y,y],[3,x,x];
  module N = [1,x,y,x],[-1,y,x,y];
  list L; L[1] = M; L[2] = N;
  lst2str(L);
  def U = crs(L,5);
  setring U; U;
  I;
  poly p = I[2]; // I[8];
  p;
  system("stest",p,7,7,3); // error -> the world is ok
  poly q1 = system("stest",p,1,7,3); //ok
  poly q6 = system("stest",p,6,7,3); //ok
  system("btest",p,3); //ok
  system("btest",q1,3); //ok
  system("btest",q6,3); //ok
}

proc ex2()
{
  option(prot);
  LIB "freegb.lib";
  ring r = 0,(x,y),dp;
  module M = [-1,x,y],[3,x,x]; // 3x^2 - xy
  def U = freegb(M,7);
  lst2str(U);
}

proc ex_nonhomog()
{
  option(prot);
  LIB "freegb.lib";
  ring r = 0,(x,y,h),dp;
  list L;
  module M;
  M = [-1,y,y],[1,x,x,x];  // x3-y2
  L[1] = M;
  M = [1,x,h],[-1,h,x];  // xh-hx
  L[2] = M;
  M = [1,y,h],[-1,h,y];  // yh-hy
  L[3] = M;
  def U = freegb(L,4);
  lst2str(U);
  // strange elements in the basis
}

proc ex_nonhomog_comm()
{
  option(prot);
  LIB "freegb.lib";
  ring r = 0,(x,y),dp;
  module M = [-1,y,y],[1,x,x,x];
  def U = freegb(M,5);
  lst2str(U);
}

proc ex_nonhomog_h()
{
  option(prot);
  LIB "freegb.lib";
  ring r = 0,(x,y,h),(a(1,1),dp);
  module M = [-1,y,y,h],[1,x,x,x]; // x3 - y2h
  def U = freegb(M,6);
  lst2str(U);
}

proc ex_nonhomog_h2()
{
  option(prot);
  LIB "freegb.lib";
  ring r = 0,(x,y,h),(dp);
  list L;
  module M;
  M = [-1,y,y,h],[1,x,x,x]; // x3 - y2h
  L[1] = M;
  M = [1,x,h],[-1,h,x]; // xh - hx
  L[2] = M;
  M = [1,y,h],[-1,h,y]; // yh - hy
  L[3] = M;
  def U = freegbasis(L,3);
  lst2str(U);
  // strange answer CHECK
}


proc ex_nonhomog_3()
{
  option(prot);
  LIB "./freegb.lib";
  ring r = 0,(x,y,z),(dp);
  list L;
  module M;
  M = [1,z,y],[-1,x]; // zy - x
  L[1] = M;
  M = [1,z,x],[-1,y]; // zx - y
  L[2] = M;
  M = [1,y,x],[-1,z]; // yx - z
  L[3] = M;
  lst2str(L);
  list U = freegb(L,4);
  lst2str(U);
  // strange answer CHECK
}

proc ex_densep_2()
{
  option(prot);
  LIB "freegb.lib";
  ring r = (0,a,b,c),(x,y),(Dp); // deglex
  module M = [1,x,x], [a,x,y], [b,y,x], [c,y,y];
  lst2str(M);
  list U = freegb(M,5);
  lst2str(U);
  // a=b is important -> finite basis!!!
  module M = [1,x,x], [a,x,y], [a,y,x], [c,y,y];
  lst2str(M);
  list U = freegb(M,5);
  lst2str(U);
}


// 1. form a new ring
// 2. produce shifted generators
// 3. compute GB
// 4. skip shifted elts
// 5. go back to orig vars, produce strings/modules
// 6. return the result

proc freegbold(list LM, int d)
"USAGE:  freegbold(L, d);  L a list of modules, d an integer
RETURN:  ring
PURPOSE: compute the two-sided Groebner basis of an ideal, encoded by L in
the free associative algebra, up to degree d
EXAMPLE: example freegbold; shows examples
"
{
  // d = up to degree, will be shifted to d+1
  if (d<1) {"bad d"; return(0);}

  int ppl = printlevel-voice+2;
  string err = "";

  int i,j,s;
  def save = basering;
  // determine max no of places in the input
  int slm = size(LM); // numbers of polys in the ideal
  int sm;
  intvec iv;
  module M;
  for (i=1; i<=slm; i++)
  {
    // modules, e.g. free polynomials
    M  = LM[i];
    sm = ncols(M);
    for (j=1; j<=sm; j++)
    {
      //vectors, e.g. free monomials
      iv = iv, size(M[j])-1; // 1 place is reserved by the coeff
    }
  }
  int D = Max(iv); // max size of input words
  if (d<D) {"bad d"; return(LM);}
  D = D + d-1;
  //  D = d;
  list LR  = ringlist(save);
  list L, tmp;
  L[1] = LR[1]; // ground field
  L[4] = LR[4]; // quotient ideal
  tmp  = LR[2]; // varnames
  s = size(LR[2]);
  for (i=1; i<=D; i++)
  {
    for (j=1; j<=s; j++)
    {
      tmp[i*s+j] = string(tmp[j])+"("+string(i+1)+")";
    }
  }
  for (i=1; i<=s; i++)
  {
    tmp[i] = string(tmp[i])+"("+string(1)+")";
  }
  L[2] = tmp;
  list OrigNames = LR[2];
  // ordering: d blocks of the ord on r
  // try to get whether the ord on r is blockord itself
  // TODO: make L(2) ordering! exponent is maximally 2
  s = size(LR[3]);
  if (s==2)
  {
    // not a blockord, 1 block + module ord
    tmp = LR[3][s]; // module ord
    for (i=1; i<=D; i++)
    {
      LR[3][s-1+i] = LR[3][1];
    }
    LR[3][s+D] = tmp;
  }
  if (s>2)
  {
    // there are s-1 blocks
    int nb = s-1;
    tmp = LR[3][s]; // module ord
    for (i=1; i<=D; i++)
    {
      for (j=1; j<=nb; j++)
      {
        LR[3][i*nb+j] = LR[3][j];
      }
    }
    //    size(LR[3]);
    LR[3][nb*(D+1)+1] = tmp;
  }
  L[3] = LR[3];
  def @R = ring(L);
  setring @R;
  ideal I;
  poly @p;
  s = size(OrigNames);
  //  "s:";s;
  // convert LM to canonical vectors (no powers)
  setring save;
  kill M; // M was defined earlier
  module M;
  slm = size(LM); // numbers of polys in the ideal
  int sv,k,l;
  vector v;
  //  poly p;
  string sp;
  setring @R;
  poly @@p=0;
  setring save;
  for (l=1; l<=slm; l++)
  {
    // modules, e.g. free polynomials
    M  = LM[l];
    sm = ncols(M); // in intvec iv the sizes are stored
    for (i=0; i<=d-iv[l]; i++)
    {
      // modules, e.g. free polynomials
      for (j=1; j<=sm; j++)
      {
        //vectors, e.g. free monomials
        v  = M[j];
        sv = size(v);
        //      "sv:";sv;
        sp = "@@p = @@p + ";
        for (k=2; k<=sv; k++)
        {
          sp = sp + string(v[k])+"("+string(k-1+i)+")*";
        }
        sp = sp + string(v[1])+";"; // coef;
        setring @R;
        execute(sp);
        setring save;
      }
      setring @R;
      //      "@@p:"; @@p;
      I = I,@@p;
      @@p = 0;
      setring save;
    }
  }
  kill sp;
  // 3. compute GB
  setring @R;
  dbprint(ppl,"computing GB");
  //  ideal J = groebner(I);
  ideal J = slimgb(I);
  dbprint(ppl,J);
  // 4. skip shifted elts
  ideal K = select1(J,1,s); // s = size(OrigNames)
  dbprint(ppl,K);
  dbprint(ppl, "done with GB");
  // K contains vars x(1),...z(1) = images of originals
  // 5. go back to orig vars, produce strings/modules
  if (K[1] == 0)
  {
    "no reasonable output, GB gives 0";
    return(0);
  }
  int sk = size(K);
  int sp, sx, a, b;
  intvec x;
  poly p,q;
  poly pn;
  // vars in 'save'
  setring save;
  module N;
  list LN;
  vector V;
  poly pn;
  // test and skip exponents >=2
  setring @R;
  for(i=1; i<=sk; i++)
  {
    p  = K[i];
    while (p!=0)
    {
      q  = lead(p);
      //      "processing q:";q;
      x  = leadexp(q);
      sx = size(x);
      for(k=1; k<=sx; k++)
      {
        if ( x[k] >= 2 )
        {
          err = "skip: the value x[k] is " + string(x[k]);
          dbprint(ppl,err);
          //        return(0);
          K[i] = 0;
          p    = 0;
          q    = 0;
          break;
        }
      }
      p  = p - q;
    }
  }
  K  = simplify(K,2);
  sk = size(K);
  for(i=1; i<=sk; i++)
  {
    //    setring save;
    //    V  = 0;
    setring @R;
    p  = K[i];
    while (p!=0)
    {
      q  = lead(p);
      err =  "processing q:" + string(q);
      dbprint(ppl,err);
      x  = leadexp(q);
      sx = size(x);
      pn = leadcoef(q);
      setring save;
      pn = imap(@R,pn);
      V  = V + leadcoef(pn)*gen(1);
      for(k=1; k<=sx; k++)
      {
        if (x[k] ==1)
        {
          a = k / s; // block number=a+1, a!=0
          b = k % s; // remainder
          //      printf("a: %s, b: %s",a,b);
          if (b == 0)
          {
            // that is it's the last var in the block
            b = s;
            a = a-1;
          }
          V = V + var(b)*gen(a+2);
        }
//      else
//      {
//        printf("error: the value x[k] is %s", x[k]);
//        return(0);
//      }
      }
      err = "V: " + string(V);
      dbprint(ppl,err);
      //      printf("V: %s", string(V));
      N = N,V;
      V  = 0;
      setring @R;
      p  = p - q;
      pn = 0;
    }
    setring save;
    LN[i] = simplify(N,2);
    N     = 0;
  }
  setring save;
  return(LN);
}
example
{
  "EXAMPLE:"; echo = 2;
  ring r = 0,(x,y,z),(dp(1),dp(2));
  module M = [-1,x,y],[-7,y,y],[3,x,x];
  module N = [1,x,y,x],[-1,y,x,y];
  list L; L[1] = M; L[2] = N;
  lst2str(L);
  def U = freegbold(L,5);
  lst2str(U);
}

proc sgb(ideal I, int d)
{
  // new code
  // map x_i to x_i(1) via map()
  //LIB "freegb.lib";
  def save = basering;
  //int d =7;// degree
  int nv = nvars(save);
  def R = freegbRing(d);
  setring R;
  int i;
  ideal Imap;
  for (i=1; i<=nv; i++)
  {
    Imap[i] = var(i);
  }
  //ideal I = x(1)*y(2), y(1)*x(2)+z(1)*z(2);
  ideal I = x(1)*x(2),x(1)*y(2) + z(1)*x(2);
  option(prot);
  //option(teach);
  ideal J = system("freegb",I,d,nv);
}



static proc checkCeq()
{
  ring r = 0,(x,y),Dp;
  def A = freegbRing(4);
  setring A;
  A;
  // I = x2-xy
  ideal I = x(1)*x(2) - x(1)*y(2), x(2)*x(3) - x(2)*y(3), x(3)*x(4) - x(3)*y(4);
  ideal C = x(2)-x(1),x(3)-x(2),x(4)-x(3),y(2)-y(1),y(3)-y(2),y(4)-y(3);
  ideal K = I,C;
  groebner(K);
}


proc exHom1()
{
  // we start with
  // z*y - x, z*x - y, y*x - z
  LIB "freegb.lib";
  LIB "elim.lib";
  ring r = 0,(x,y,z,h),dp;
  list L;
  module M;
  M = [1,z,y],[-1,x,h]; // zy - xh
  L[1] = M;
  M = [1,z,x],[-1,y,h]; // zx - yh
  L[2] = M;
  M = [1,y,x],[-1,z,h]; // yx - zh
  L[3] = M;
  lst2str(L);
  def U = crs(L,4);
  setring U;
  I = I, 
    y(2)*h(3)+z(2)*x(3),     y(3)*h(4)+z(3)*x(4), 
    y(2)*x(3)-z(2)*h(3),     y(3)*x(4)-z(3)*h(4);
  I = simplify(I,2);
  ring r2 = 0,(x(0..4),y(0..4),z(0..4),h(0..4)),dp;
  ideal J = imap(U,I);
  //  ideal K = homog(J,h);
  option(redSB);
  option(redTail);
  ideal L = groebner(J); //(K);
  ideal LL = sat(L,ideal(h))[1];
  ideal M = subst(LL,h,1);
  M = simplify(M,2);
  setring U;
  ideal M = imap(r2,M);
  lst2str(U);
}

static proc test1()
{
  LIB "freegb.lib";
  ring r = 0,(x,y),Dp;
  int d = 10; // degree
  def R = freegbRing(d);
  setring R;
  ideal I = x(1)*x(2) - y(1)*y(2);
  option(prot);
  option(teach);
  ideal J = system("freegb",I,d,2);
  J;
}

static proc test2()
{
  LIB "freegb.lib";
  ring r = 0,(x,y),Dp;
  int d = 10; // degree
  def R = freegbRing(d);
  setring R;
  ideal I = x(1)*x(2) - x(1)*y(2);
  option(prot);
  option(teach);
  ideal J = system("freegb",I,d,2);
  J;
}

static proc test3()
{
  LIB "freegb.lib";
  ring r = 0,(x,y,z),dp;
  int d =5; // degree
  def R = freegbRing(d);
  setring R;
  ideal I = x(1)*y(2), y(1)*x(2)+z(1)*z(2);
  option(prot);
  option(teach);
  ideal J = system("freegb",I,d,3);
}

proc schur2-3()
{
  // nonhomog:
  //  h^4-10*h^2+9,f*e-e*f+h, h*2-e*h-2*e,h*f-f*h+2*f
  // homogenized with t
  //  h^4-10*h^2*t^2+9*t^4,f*e-e*f+h*t, h*2-e*h-2*e*t,h*f-f*h+2*f*t,
  // t*h - h*t, t*f - f*t, t*e - e*t
}