Function idElimination

kernel/ideals.cc:1605–1847

2 * eliminate delVar (product of vars) in h1 */

* eliminate delVar (product of vars) in h1
*/
ideal idElimination (ideal h1,poly delVar,intvec *hilb, GbVariant alg)
{
  int    i,j=0,k,l;
  ideal  h,hh, h3;
  rRingOrder_t    *ord;
  int    *block0,*block1;
  int    ordersize=2;
  int    **wv;
  tHomog hom;
  intvec * w;
  ring tmpR;
  ring origR = currRing;

  if (delVar==NULL)
  {
    return idCopy(h1);
  }
  if ((currRing->qideal!=NULL) && rIsPluralRing(origR))
  {
    WerrorS("cannot eliminate in a qring");
    return NULL;
  }
  if (idIs0(h1)) return idInit(1,h1->rank);
#ifdef HAVE_PLURAL
  if (rIsPluralRing(origR))
    /* in the NC case, we have to check the admissibility of */
    /* the subalgebra to be intersected with */
  {
    if ((ncRingType(origR) != nc_skew) && (ncRingType(origR) != nc_exterior)) /* in (quasi)-commutative algebras every subalgebra is admissible */
    {
      if (nc_CheckSubalgebra(delVar,origR))
      {
        WerrorS("no elimination is possible: subalgebra is not admissible");
        return NULL;
      }
    }
  }
#endif
  hom=(tHomog)idHomModule(h1,NULL,&w); //sets w to weight vector or NULL
  h3=idInit(16,h1->rank);
  ordersize=rBlocks(origR)+1;
#if 0
  if (rIsPluralRing(origR)) // we have too keep the odering: it may be needed
                            // for G-algebra
  {
    for (k=0;k<ordersize-1; k++)
    {
      block0[k+1] = origR->block0[k];
      block1[k+1] = origR->block1[k];
      ord[k+1] = origR->order[k];
      if (origR->wvhdl[k]!=NULL) wv[k+1] = (int*) omMemDup(origR->wvhdl[k]);
    }
  }
  else
  {
    block0[1] = 1;
    block1[1] = (currRing->N);
    if (origR->OrdSgn==1) ord[1] = ringorder_wp;