// $Id: elim.lib,v 1.11 2000-12-22 13:42:34 greuel Exp $
// (GMG, last modified 22.06.96)
///////////////////////////////////////////////////////////////////////////////
version="$Id: elim.lib,v 1.11 2000-12-22 13:42:34 greuel Exp $";
category="Commutative Algebra";
info="
LIBRARY:  elim.lib      Elimination, Saturation and Blowing up

PROCEDURES:
 blowup0(j[,s1,s2]);    create presentation of blownup ring of ideal j
 elim(id,n,m);          variable n..m eliminated from id (ideal/module)
 elim1(id,p);           p=product of vars to be eliminated from id
 nselect(id,n[,m]);     select generators not containing nth [..mth] variable
 sat(id,j);             saturated quotient of ideal/module id by ideal j
 select(id,n[,m]);      select generators containing all variables n...m
 select1(id,n[,m]);     select generators containing one variable n...m
           (parameters in square brackets [] are optional)
";

LIB "inout.lib";
LIB "general.lib";
LIB "poly.lib";
///////////////////////////////////////////////////////////////////////////////

proc blowup0 (ideal j,list #)
"USAGE:   blowup0(j[,s1,s2]); j ideal, s1,s2 nonempty strings
CREATE:  Create a presentation of the blowup ring of j
RETURN:  no return value
NOTE:    s1 and s2 are used to give names to the blownup ring and the blownup
         ideal (default: s1=\"j\", s2=\"A\")
         Assume R = char,x(1..n),ord is the basering of j, and s1=\"j\", s2=\"A\"
         then the procedure creates a new ring with name Bl_jR
         (equal to R[A,B,...])
               Bl_jR = char,(A,B,...,x(1..n)),(dp(k),ord)
         with k=ncols(j) new variables A,B,... and ordering wp(d1..dk) if j is
         homogeneous with deg(j[i])=di resp. dp otherwise for these vars.
         If k>26 or size(s2)>1, say s2=\"A()\", the new vars are A(1),...,A(k).
         Let j_ be the kernel of the ring map Bl_jR -> R defined by A(i)->j[i],
         x(i)->x(i), then the quotient ring Bl_jR/j_ is the blowup ring of j
         in R (being isomorphic to R+j+j^2+...). Moreover the procedure creates
         a std basis of j_ with name j_ in Bl_jR.
         This proc uses 'execute' or calls a procedure using 'execute'.
DISPLAY: printlevel >=0: explain created objects (default)
EXAMPLE: example blowup0; shows examples
"{
   string bsr = nameof(basering);
   def br = basering;
   string cr,vr,o = charstr(br),varstr(br),ordstr(br);
   int n,k,i = nvars(br),ncols(j),0;
   int p = printlevel-voice+3;  // p=printlevel+1 (default: p=1)
//---------------- create coordinate ring of blown up space -------------------
   if( size(#)==0 ) { #[1] = "j"; #[2] = "A"; }
   if( size(#)==1 ) { #[2] = "A"; }
   if( k<=26 and size(#[2])==1 ) { string nv = A_Z(#[2],k)+","; }
   else { string nv = (#[2])[1]+"(1.."+string(k)+"),"; }
   if( is_homog(j) )
   {
      intvec v=1;
      for( i=1; i<=k; i=i+1) { v[i+1]=deg(j[i]); }
      string nor = "),(wp(v),";
   }
   else { string nor = "),(dp(1+k),";}
   execute("ring Bl=("+cr+"),(t,"+nv+vr+nor+o+");");
//---------- map to new ring, eliminate and create blown up ideal -------------
   ideal j=imap(br,j);
   for( i=1; i<=k; i=i+1) { j[i]=var(1+i)-t*j[i]; }
   j=eliminate(j,t);
   v=v[2..size(v)];
   execute("ring Bl_"+#[1]+bsr+"=("+cr+"),("+nv+vr+nor+o+");");
   ideal `#[1]+"_"`=imap(Bl,j);
   export basering;
   export `#[1]+"_"`;
   //keepring basering;
   setring br;
//------------------- some comments about usage and names  --------------------
dbprint(p,"",
"// The proc created the ring Bl_"+#[1]+bsr+" (equal to "+bsr+"["+nv[1,size(nv)-1]+"])",
"// it contains the ideal "+#[1]+"_ , such that",
"//             Bl_"+#[1]+bsr+"/"+#[1]+"_ is the blowup ring",
"// show(Bl_"+#[1]+bsr+"); shows this ring.",
"// Make Bl_"+#[1]+bsr+" the basering and see "+#[1]+"_ by typing:",
"   setring Bl_"+#[1]+bsr+";","   "+#[1]+"_;");
   return();
}
example
{ "EXAMPLE:"; echo = 2;
   ring R=0,(x,y),dp;
   poly f=y2+x3; ideal j=jacob(f);
   blowup0(j);
   show(Bl_jR);
   setring Bl_jR;
   j_;"";
   ring r=32003,(x,y,z),ds;
   blowup0(maxideal(1),"m","T()");
   show(Bl_mr);
   setring Bl_mr;
   m_;
   kill Bl_jR, Bl_mr;
}
///////////////////////////////////////////////////////////////////////////////

proc elim (id, int n, int m)
"USAGE:   elim(id,n,m);  id ideal/module, n,m integers
RETURNS: ideal/module obtained from id by eliminating variables n..m
NOTE:    no special monomial ordering is required, result is a SB with
         respect to ordering dp (resp. ls) if the first var not to be
         eliminated belongs to a -p (resp. -s) blockordering
         This proc uses 'execute' or calls a procedure using 'execute'.
EXAMPLE: example elim; shows examples
"
{
//---- get variables to be eliminated and create string for new ordering ------
   int ii; poly vars=1;
   for( ii=n; ii<=m; ii=ii+1 ) { vars=vars*var(ii); }
   if( n>1 ) { poly p = 1+var(1); }
   else { poly p = 1+var(m+1); }
   if( ord(p)==0 ) { string ordering = "),ls;"; }
   if( ord(p)>0 ) { string ordering = "),dp;"; }
   string mpoly=string(minpoly);
//-------------- create new ring and map objects to new ring ------------------
   def br = basering;
   string str = "ring @newr = ("+charstr(br)+"),("+varstr(br)+ordering;
   execute(str);
   if (mpoly!="0") { execute("minpoly="+mpoly+";"); }
   def i = imap(br,id);
   poly vars = imap(br,vars);
//---------- now eliminate in new ring and map back to old ring ---------------
   i = eliminate(i,vars);
   setring br;
   return(imap(@newr,i));
}
example
{ "EXAMPLE:"; echo = 2;
   ring r=0,(x,y,u,v,w),dp;
   ideal i=x-u,y-u2,w-u3,v-x+y3;
   elim(i,3,4);
   module m=i*gen(1)+i*gen(2);
   m=elim(m,3,4);show(m);
}
///////////////////////////////////////////////////////////////////////////////

proc elim1 (id, poly vars)
"USAGE:   elim1(id,p); id ideal/module, p product of vars to be eliminated
RETURN:  ideal/module obtained from id by eliminating vars occuring in poly
NOTE:    no special monomial ordering is required, result is a SB with
         respect to ordering dp (resp. ls) if the first var not to be
         eliminated belongs to a -p (resp. -s) blockordering
         This proc uses 'execute' or calls a procedure using 'execute'.
EXAMPLE: example elim1; shows examples
"
{
//---- get variables to be eliminated and create string for new ordering ------
   int ii;
   for( ii=1; ii<=nvars(basering); ii=ii+1 )
   {
      if( vars/var(ii)==0 ) { poly p = 1+var(ii); break;}
   }
   if( ord(p)==0 ) { string ordering = "),ls;"; }
   if( ord(p)>0 ) { string ordering = "),dp;"; }
//-------------- create new ring and map objects to new ring ------------------
   def br = basering;
   string str = "ring @newr = ("+charstr(br)+"),("+varstr(br)+ordering;
   execute(str);
   def id = fetch(br,id);
   poly vars = fetch(br,vars);
//---------- now eliminate in new ring and map back to old ring ---------------
   id = eliminate(id,vars);
   setring br;
   return(imap(@newr,id));
}
example
{ "EXAMPLE:"; echo = 2;
   ring r=0,(x,y,t,s,z),dp;
   ideal i=x-t,y-t2,z-t3,s-x+y3;
   elim1(i,ts);
   module m=i*gen(1)+i*gen(2);
   m=elim1(m,st); show(m);
}
///////////////////////////////////////////////////////////////////////////////

proc nselect (id, int n, list#)
"USAGE:   nselect(id,n[,m]); id a module or ideal, n, m integers
RETURN:  generators of id not containing the variable n [up to m]
EXAMPLE: example nselect; shows examples
"{
   int j,k;
   if( size(#)==0 ) { #[1]=n; }
   for( k=1; k<=ncols(id); k=k+1 )
   {  for( j=n; j<=#[1]; j=j+1 )
      {  if( size(id[k]/var(j))!=0) { id[k]=0; break; }
      }
   }
   return(simplify(id,2));
}
example
{ "EXAMPLE:"; echo = 2;
   ring r=0,(x,y,t,s,z),(c,dp);
   ideal i=x-y,y-z2,z-t3,s-x+y3;
   nselect(i,3);
   module m=i*(gen(1)+gen(2));
   show(m);
   show(nselect(m,3,4));
}
///////////////////////////////////////////////////////////////////////////////

proc sat (id, ideal j)
"USAGE:   sat(id,j);  id=ideal/module, j=ideal
RETURN:  list of an ideal/module [1] and an integer [2]:
         [1] = saturation of id with respect to j (= union_(k=1...) of id:j^k)
         [2] = saturation exponent (= min( k | id:j^k = id:j^(k+1) ))
NOTE:    [1] is a standard basis in the basering
DISPLAY: saturation exponent during computation if printlevel >=1
EXAMPLE: example sat; shows an example
"{
   int ii,kk;
   def i=id;
   id=std(id);
   int p = printlevel-voice+3;  // p=printlevel+1 (default: p=1)
   while( ii<=size(i) )
   {
      dbprint(p-1,"// compute quotient "+string(kk+1));
      i=quotient(id,j);
      for( ii=1; ii<=size(i); ii=ii+1 )
      {
         if( reduce(i[ii],id,1)!=0 ) break;
      }
      id=std(i); kk=kk+1;
   }
   dbprint(p-1,"// saturation becomes stable after "+string(kk-1)+" iteration(s)","");
   list L = id,kk-1;
   return (L);
}
example
{ "EXAMPLE:"; echo = 2;
   int p      = printlevel;
   ring r     = 2,(x,y,z),dp;
   poly F     = x5+y5+(x-y)^2*xyz;
   ideal j    = jacob(F);
   sat(j,maxideal(1));
   printlevel = 2;
   sat(j,maxideal(2));
   printlevel = p;
}
///////////////////////////////////////////////////////////////////////////////

proc select (id, int n, list#)
"USAGE:   select(id,n[,m]); id ideal/module, n, m integers
RETURN:  generators of id containing the variable n [all variables up to m]
NOTE:    use 'select1' for selecting generators containing at least one of the
         variables between n and m
EXAMPLE: example select; shows examples
"{
   if( size(#)==0 ) { #[1]=n; }
   int j,k;
   for( k=1; k<=ncols(id); k=k+1 )
   {  for( j=n; j<=#[1]; j=j+1 )
      {   if( size(id[k]/var(j))==0) { id[k]=0; break; }
      }
   }
   return(simplify(id,2));
}
example
{ "EXAMPLE:"; echo = 2;
   ring r=0,(x,y,t,s,z),(c,dp);
   ideal i=x-y,y-z2,z-t3,s-x+y3;
   ideal j=select(i,1);
   j;
   module m=i*(gen(1)+gen(2));
   m;
   select(m,1,2);
}
///////////////////////////////////////////////////////////////////////////////

proc select1 (id, int n, list#)
"USAGE:   select(id,n[,m]); id ideal/module, n, m integers
RETURN:  generators of id containing the variable n 
         [at least one of the variables up to m]
NOTE:    use 'select' for selecting generators containing all the
         variables between n and m
EXAMPLE: example select1; shows examples
"{
   if( size(#)==0 ) { #[1]=n; }
   int j,k;
   execute (typeof(id)+" I;");
   for( k=1; k<=ncols(id); k=k+1 )
   {  for( j=n; j<=#[1]; j=j+1 )
      {   
         if( size(subst(id[k],var(j),0)) != size(id[k]) ) 
         { I=I,id[k]; break; }
      }
   }
   return(simplify(I,2));
}
example
{ "EXAMPLE:"; echo = 2;
   ring r=0,(x,y,t,s,z),(c,dp);
   ideal i=x-y,y-z2,z-t3,s-x+y3;
   ideal j=select1(i,1);
   j;
   module m=i*(gen(1)+gen(2));
   m;
   select1(m,1,2);
}
///////////////////////////////////////////////////////////////////////////////