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// egr.cc: implementation of functions for reduction of points & component groups
//////////////////////////////////////////////////////////////////////////
//
// Copyright 1990-2012 John Cremona
//
// This file is part of the eclib package.
//
// eclib is free software; you can redistribute it and/or modify it
// under the terms of the GNU General Public License as published by the
// Free Software Foundation; either version 2 of the License, or (at your
// option) any later version.
//
// eclib is distributed in the hope that it will be useful, but WITHOUT
// ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
// FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
// for more details.
//
// You should have received a copy of the GNU General Public License
// along with eclib; if not, write to the Free Software Foundation,
// Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA
//
//////////////////////////////////////////////////////////////////////////
#include <eclib/points.h>
#include <eclib/egr.h>
#include <eclib/matrix.h>
//#define DEBUG_EGR
//#define DEBUG_EGR_EXTRA
// return 1 if P mod p is nonsingular:
int ComponentGroups::HasGoodReduction(const Point& P, const bigint& p)
{
#ifdef DEBUG_EGR
cout<<"Testing whether point "
<<P
<<" has good reduction at "<<p<<"..."<<flush;
#endif
bigint Z=getZ(P);
if(is_zero(Z)) // identity is nonsingular
{
#ifdef DEBUG_EGR
cout<<"yes (P=identity)"<<endl;
#endif
return 1;
}
bigint X=getX(P);
bigint Y=getY(P);
if(is_zero(p)) // test whether P is not on the "egg"
{
if(conncomp==1)
{
#ifdef DEBUG_EGR
cout<<"yes (only one component)"<<endl;
#endif
return 1;
}
bigint fd = 6*X*X+b2*X*Z+b4*Z*Z;
if(sign(fd)<0)
{
#ifdef DEBUG_EGR
cout<<"no (real f' condition)"<<endl;
#endif
return 0;
}
bigint fdd = 12*X+b2*Z;
if(sign(fdd)<0) // assumes Z>0
{
#ifdef DEBUG_EGR
cout<<"no (real f\" condition)"<<endl;
#endif
return 0;
}
#ifdef DEBUG_EGR
cout<<"yes (real, on identity component)"<<endl;
#endif
return 1;
}
X=mod(X,p); Y=mod(Y,p); Z=mod(Z,p);
if(is_zero(Z)) // identity is nonsingular
{
#ifdef DEBUG_EGR
cout<<"yes (identity mod p)"<<endl;
#endif
return 1;
}
if(ndiv(p,-3*X*X - 2*a2*X*Z + a1*Y*Z - a4*Z*Z))
{
#ifdef DEBUG_EGR
cout<<"yes (FX nonzero mod p)"<<endl;
#endif
return 1;
}
if(ndiv(p,a1*X + 2*Y + a3*Z))
{
#ifdef DEBUG_EGR
cout<<"yes (FY nonzero mod p)"<<endl;
#endif
return 1;
}
if(ndiv(p,-a2*X*X + a1*X*Y - 2*a4*X*Z + Y*Y + 2*a3*Y*Z - 3*a6*Z*Z))
{
#ifdef DEBUG_EGR
cout<<"yes (FZ nonzero mod p)"<<endl;
#endif
return 1;
}
#ifdef DEBUG_EGR
cout<<"no"<<endl;
#endif
return 0;
}
// return 1 if P mod p is nonsingular for all p in plist; else return
// 0 and put the first prime of bad reduction into p0:
int ComponentGroups::HasGoodReduction(const Point& P, const vector<bigint>& plist, bigint& p0)
{
for(unsigned int i=0; i<plist.size(); i++)
{p0=plist[i]; if(!HasGoodReduction(P,p0)) return 0;}
return 1;
}
// return 1 iff P mod p is nonsingular for all p (including infinity);
// else return 0 and put the first prime of bad reduction into p0:
int ComponentGroups::HasGoodReduction(const Point& P, bigint& p0)
{
if(!HasGoodReduction(P,BIGINT(0))) {p0=BIGINT(0); return 0;}
return HasGoodReduction(P,the_bad_primes,p0);
}
// Returns [m] for cyclic of order m, [2,2] for 2*2 (type I*m, m even)
vector<int> ComponentGroups::ComponentGroup(const bigint& p)
{
vector<int> ans(1);
if(p==0) ans[0]=conncomp; // 1 or 2
else
{
ans[0]=1;
map<bigint,Reduction_type>::const_iterator ri = reduct_array.find(p);
if(ri==reduct_array.end()) return ans; // p has good reduction
ans[0] = (ri->second).c_p; // usual case: cyclic of order cp
int code=(ri->second).Kcode.code;
if((code%10==1)&&even((code-1)/10)) // Type I*m, m even: [2,2]
{ans[0]=2; ans.push_back(2);}
}
return ans;
}
// Returns 1 iff P and Q have same image in the component group at p:
//
int ComponentGroups::InSameComponent(const Point& P, const Point& Q, const bigint& p)
{
if(P==Q) return 1;
return HasGoodReduction(P-Q,p);
}
// For reduction type Im, multiplicative reduction where component
// group is cyclic of order m, using Tate curve formula from Silverman
// Returns a such that P mod pr maps to +a or -a mod m in the
// component group
long ComponentGroups::ImageInComponentGroup_Im_pm(const Point&P, const bigint& p, int m)
{
#ifdef DEBUG_EGR
cout<<"In ImageInComponentGroup_Im_pm() with point "
<<P
<<", m="<<m<<"..."<<flush;
#endif
if(HasGoodReduction(P,p)) return 0;
bigint x=getX(P), y=getY(P), z=getZ(P);
bigint zroot = gcd(x,z); // = cube root of z
long ans = val(p, 2*y + a1*x + a3*z) - 3*val(p,zroot);
#ifdef DEBUG_EGR
cout<<"ImageInComponentGroup_Im_pm() first gives ans = "<<ans<<endl;
#endif
if(even(m)&&(ans>(m/2))) ans=m/2;
#ifdef DEBUG_EGR
cout<<"ImageInComponentGroup_Im_pm() returns "<<ans<<endl;
#endif
return ans;
}
// For reduction type Im, multiplicative reduction where component
// group is cyclic of order m, using full Tate curve formula.
// Returns a such that P mod pr maps to a mod m in the component group
long ComponentGroups::ImageInComponentGroup_Im(const Point&P, const bigint& p, int m)
{
#ifdef DEBUG_EGR
cout<<"In ImageInComponentGroup_Im() with point "
<<P
<<", m="<<m<<"..."<<flush;
#endif
if(HasGoodReduction(P,p)) return 0;
// The following is independent of P
long N = m; // to match the write-up
// long N2=(N+1)/2; // =N/2 rounded up
long N2=N;
bigint pN = pow(p,N2);
bigint c4inv = invmod(c4,pN);
bigint x0 = c4inv*(18*b6-b2*b4);
bigint y0 = c4inv*(a1*a1*a1*a4 - 2*a1*a1*a2*a3 + 4*a1*a2*a4 + 3*a1*a3*a3 - 36*a1*a6 - 8*a2*a2*a3 + 24*a3*a4);
x0 = x0 % pN;
y0 = y0 % pN;
#ifdef DEBUG_EGR_EXTRA
cout<<"c4inv = "<<c4inv<<endl;
cout<<"x0 = "<<x0<<", y0="<<y0<<endl;
#endif
bigint d2 = b2+12*x0;
bigint d; sqrt_mod_p_power(d,d2,p,N2);
bigint alpha1 = (d-a1);
if(odd(alpha1))
{
if(p==2)
cout<<"Problem in ComponentGroups::ImageInComponentGroup_Im(): "
<<"quadratic has no roots\n";
else alpha1+=pN; // saves inverting 2 mod pN
}
alpha1/=2;
bigint alpha2 = (-a1-alpha1);
#ifdef DEBUG_EGR_EXTRA
cout<<"alpha1 = "<<alpha1<<", alpha2="<<alpha2<<endl;
#endif
bigint z=getZ(P);
bigint zinv = invmod(z,pN);
bigint x=(getX(P)*zinv-x0) %pN;
bigint y=(getY(P)*zinv-y0) %pN;
bigint w1 = (y - alpha1*x) %pN;
bigint w2 = (y - alpha2*x) %pN;
long e1 = val(p, w1); if(e1>N2) e1=N2;
long e2 = val(p, w2); if(e2>N2) e2=N2;
#ifdef DEBUG_EGR_EXTRA
cout<<"x = "<<x<<", y="<<y<<endl;
cout<<"(y-y0) - alpha1*(x-x0) = "<<w1<<endl;
cout<<"(y-y0) - alpha2*(x-x0) = "<<w2<<endl;
cout<<"e1="<<e1<<", e2="<<e2<<endl;
#endif
long ans=0;
if(e1<e2)
{
if((0<e1)&&(2*e1<N)) ans=-e1;
else
cout<<"Problem! e1 not between 1 and "<<((N+1)/2)-1<<endl;
}
else
if(e2<e1)
{
if((0<e2)&&(2*e2<N)) ans=+e2;
else
cout<<"Problem! e2 not between 1 and "<<((N+1)/2)-1<<endl;
}
else
{
if(even(N)) ans=N/2;
else
cout<<"Problem! e1=e2="<<e1<<" but N="<<N<<"is not even!"<<endl;
}
#ifdef DEBUG_EGR
cout<<"ImageInComponentGroup_Im() gives ans = "<<ans<<endl;
#endif
return ans;
}
long ComponentGroups::ImageInComponentGroup(const Point&P, const bigint& p, vector<int> grp)
{
#ifdef DEBUG_EGR
cout<<"In ImageInComponentGroup() with point "
<<P
<<", p = "<<p<<", group = "<<grp<<"..."<<flush;
#endif
if(grp.size()==2) // C2xC2, cannot handle
{
cout<<"Error in ComponentGroups::ImageInComponentGroup(): noncyclic case"<<endl;
abort();
}
int ans=0; // the default
long n=grp[0]; // the group is cyclic of order n
switch(n) {
case 1: {break;}
case 2: {
if(!HasGoodReduction(P,p))
{ans=1; }
break;}
case 3: {
if(!HasGoodReduction(P,p))
{ans=1; }
break;}
case 4: {
if(!HasGoodReduction(P,p))
{
if(HasGoodReduction(2*P,p))
{ans=2; }
else
{ans=1; }
}
break;}
default:
// Now we are in the case of I_n
{
ans= ImageInComponentGroup_Im(P,p,n);
#if(0) // testing only
for(int i=0; i<n; i++)
{
int t = (i*ans-ImageInComponentGroup_Im(i*P,p,n)) % n;
if(t) cout<<"Problem!"<<endl;
}
#endif
}
}
#ifdef DEBUG_EGR
cout<<"ImageInComponentGroup() returns "<<ans<<endl;
#endif
return ans;
}
//#undef DEBUG_EGR
//#define DEBUG_EGR
// Return least j>0 such that j*P has good reduction at p; the
// component group order is given so we only test j dividing this
//
// Since ComponentGroups are small we use nothing fancy here...
int ComponentGroups::OrderInComponentGroup(const Point& P, const bigint& p, vector<int> grp)
{
#ifdef DEBUG_EGR
cout<<"In OrderInComponentGroup() with point "
<<P
<<", p = "<<p<<", group = "<<grp<<"..."<<flush;
#endif
int ans=1;
long n=grp[0]; // the group order
if(grp.size()==2) // C2xC2
{
if(!HasGoodReduction(P,p)) ans=2;
}
else // cyclic of order n
{
ans = n/gcd(n,ImageInComponentGroup(P,p,grp));
}
#ifdef DEBUG_EGR
cout<<"OrderInComponentGroup() returns "<<ans<<endl;
#endif
return ans;
}
//#define DEBUG_EGR
// replace (independent) points in Plist with a new set which spans
// the subgroup of the original with good reduction at p, returning
// the index
int ComponentGroups::gr1prime(vector<Point>& Plist, const bigint& p)
{
int j,k,m,n,n0,n1,npts=Plist.size();
#ifdef DEBUG_EGR
cout<<"in gr1prime with p="<<p<<endl;
// bigfloat reg0 = regulator(Plist);
// cout<<"regulator = "<<reg0<<endl;
// cout<<n<<" points"<<endl;
#endif
if(npts==0) return 1;
Point P0 = Plist[0], P1;
vector<int> CG = ComponentGroup(p);
long CGexpo=CG[0];
long CGOrder=CGexpo;
if(CG.size()>1) CGOrder*=CG[1]; // =4
#ifdef DEBUG_EGR
cout<<"Component group structure = "<<CG<<endl;
#endif
if(CG.size()>1)
{
#ifdef DEBUG_EGR
cout<<"Non-cyclic component group "<<endl;
#endif
int logm=0; // = no. of gens so far
m = 1; // = order so far = 2^logm
Point Third; // will hold sum of first two gens
for (k=0; k<npts; k++)
{
Point Pk=Plist[k];
#ifdef DEBUG_EGR
cout<< "processing point#"<<k<<": "<<Pk<<endl;
#endif
j=OrderInComponentGroup(Pk,p,CG);
#ifdef DEBUG_EGR
cout<< "image has order "<<j<<" in component group"<<endl;
#endif
if (2%j)
{
cout<< "Error: order of "<<Pk
<<" in the component group is not 1 or 2!"<<endl;
abort();
}
if (j==1) continue; // good reduction, nothing to do
switch(logm)
{
case 0: // no generators yet, we have the first generator
{
if(k>0) // Make this the first point in list
{
Plist[k]=Plist[0];
Plist[0]=Pk;
Pk=Plist[k];
}
logm+=1; m*=2;
break;
} // end of case 0
case 1: // one generator so far: do we have a second generator?
{
if (InSameComponent(Pk,Plist[0],p)) // no
{
Plist[k]=Pk-Plist[0];
}
else // yes
{
if(k>1) // Make this the second point in list
{
Plist[k]=Plist[1];
Plist[1]=Pk;
Pk=Plist[k];
}
Third = Plist[0]+Plist[1];
logm+=1; m*=2;
}
break;
} // end of case 1
case 2: // we already have 2 generators
{
if (InSameComponent(Pk,Plist[0],p))
{
Plist[k]=Pk-Plist[0];
}
else
if (InSameComponent(Pk,Plist[1],p))
{
Plist[k]=Pk-Plist[1];
}
else
{
if (InSameComponent(Pk,Third,p))
{
Plist[k]=Pk-Third;
}
else
{
cout<<"Problem in non-cyclic component group case!"<<endl;
abort();
}
}
} // end of case 2
} // end of switch on logm
} // end of loop on points
if(logm>0) Plist[0]=2*Plist[0];
if(logm>1) Plist[1]=2*Plist[1];
return m;
} // end of cyclic switch
#ifdef DEBUG_EGR
cout<<"Cyclic component group of order "<<CGOrder<<endl;
#endif
if(CGOrder==1) return 1;
#ifdef DEBUG_EGR
cout<< "processing point #1: "<<flush;
#endif
m=OrderInComponentGroup(P0,p,CG); // will hold current index
for(j=1; j<npts; j++)
{
n0 = m;
P0=Plist[0]; // NB this will change
P1=Plist[j];
#ifdef DEBUG_EGR
cout<< "processing point #"<<(j+1)<<": "<<flush;
#endif
n1 = OrderInComponentGroup(P1,p,CG);
if(n1==1)
{
#ifdef DEBUG_EGR
cout<< "point has good reduction, no action needed "<<endl;
#endif
continue;
}
#ifdef DEBUG_EGR
cout<< "image has order "<<n1<<" in component group"<<endl;
#endif
// swap points so n0>=n1 for convenience
if(n1>n0)
{
#ifdef DEBUG_EGR
cout<<" swapping "<<P0<<" and "<<P1<<endl;
#endif
Point Q=P0; P0=P1; P1=Q;
n=n0; n0=n1; n1=n; m=n0;
}
if((n0%n1)==0) // P1 is a multiple of P0 in CG...
{
while((!HasGoodReduction(P1,p))) {P1=P1-P0;}
#ifdef DEBUG_EGR
cout<<"P1 replaced by "<<P1<<" with good reduction"<<endl;
cout<<"index is now "<<m<<endl;
#endif
Plist[0]=P0;
Plist[j]=P1;
}
else // lcm(n0,n1)>n0: we gain something
{
long a,b,g=gcd(n0,n1);
// Now find u (coprime to g) s.t. (n1/g)P1 == u* (n0/g)P0
Point Q0=(n0/g)*P0; Point Q=Q0; // (P0.getcurve());
Point Q1=(n1/g)*P1;
long u=1; //0;
#ifdef DEBUG_EGR
cout<<"Looking for u"<<endl;
#endif
while(gcd(u,CGOrder)>1 || (!InSameComponent(Q,Q1,p))) {u++; Q=Q+Q0;}
#ifdef DEBUG_EGR
cout<<" u="<<u<<endl;
#endif
g=bezout(u*n0,n1,a,b);
#ifdef DEBUG_EGR
cout<<" a="<<a<<", b="<<b<<", g="<<g<<endl;
#endif
Point newP0 = b*P0+a*P1;
P1=(-u*n0/g)*P0+(n1/g)*P1;
P0=newP0;
#ifdef DEBUG_EGR
cout<<"new index = lcm="<<((n0*n1)/g)<<endl;
#endif
m= ((n0*n1)/g);
Plist[0]=P0; // this now has index m
Plist[j]=P1; // this has good reduction
}
}
// At this point all points after Plist[0] have good reduction, and
// Plist[0] has index m
Plist[0]=m*Plist[0];
#ifdef DEBUG_EGR
cout<<" gr1prime returns index "<<m<<", points "<<Plist<<endl;
// bigfloat reg1 = regulator(Plist);
// cout<<" new regulator = "<<reg1<<endl;
// cout<<" ratio = "<<reg1/reg0<<endl;
#endif
return m;
}
// replaces the (independent) points with a new set which spans the
// subgroup of the original with good reduction at all p in plist,
// returning the overall index
int ComponentGroups::grprimes(vector<Point>& Plist, const vector<bigint>& plist)
{
#ifdef DEBUG_EGR
cout<<"in grprimes with plist="<<plist<<endl;
#endif
int m=1;
int n=Plist.size();
if(n>0)
for(vector<bigint>::const_iterator pj=plist.begin(); pj!=plist.end(); pj++)
m*=gr1prime(Plist,*pj);
#ifdef DEBUG_EGR
cout<<" grprimes returns index "<<m<<endl;
#endif
return m;
}
// replaces the (independent) points with a new set which spans the
// subgroup of the original with good reduction at all p,
// returning the overall index
int ComponentGroups::egr_subgroup(vector<Point>& Plist, int real_too)
{
if(Plist.size()==0) return 1;
vector<bigint> plist = the_bad_primes;
if(real_too && (conncomp==2)) plist.push_back(BIGINT(0));
#ifdef DEBUG_EGR
cout<<"Using primes "<<plist<<endl;
#endif
return grprimes(Plist,plist);
}
bigint comp_map_image(const vector<int> moduli, const mat& image);
bigint egr_index(const vector<Point>& Plist, int real_too)
{
if(Plist.size()==0) return BIGINT(1);
ComponentGroups CGS(Plist[0].getcurve());
vector<bigint> plist = getbad_primes(CGS);
if(real_too && (getconncomp(CGS)==2)) plist.push_back(BIGINT(0));
#ifdef DEBUG_EGR
cout<<"Using primes "<<plist<<endl;
#endif
vector<vector<vector<int> > > imagematrix;
vector<int> moduli;
int n=0;
for(vector<bigint>::const_iterator pi=plist.begin(); pi!=plist.end(); pi++)
{
#ifdef DEBUG_EGR
cout<<"p = "<<(*pi)<<endl;
#endif
vector<vector<int> > im=MapPointsToComponentGroup(CGS,Plist,*pi);
#ifdef DEBUG_EGR
cout<<"image = ";
for(unsigned int j=0; j<im.size(); j++) cout << im[j] << " ";
cout<<endl;
#endif
imagematrix.push_back(im);
vector<int> CG=CGS.ComponentGroup(*pi);
for(unsigned int ni=0; ni<CG.size(); ni++, n++)
moduli.push_back(CG[ni]);
}
mat m(Plist.size(),n);
unsigned int j=0, j1, j2, i;
bigint imageorder=BIGINT(1);
for(i=0; i<moduli.size(); i++) imageorder*=moduli[i];
for(j1=0; j1<plist.size(); j1++)
for(j2=0; j2<imagematrix[j1][0].size(); j2++)
{
for(i=0; i<Plist.size(); i++)
m(i+1,j+1)=imagematrix[j1][i][j2];
j++;
}
#ifdef DEBUG_EGR
cout<<"Moduli: = "<<moduli<<endl;
cout<<"Image matrix = "<<m<<endl;
cout<<"Maximum image order = "<<imageorder<<endl;
#endif
imageorder = comp_map_image(moduli,m);
#ifdef DEBUG_EGR
cout<<"Actual image order = "<<imageorder<<endl;
#endif
return imageorder;
}
// Given a list of points P1,...,Pn and a prime p, this returns a
// vector [c1,c2,...,cn] where the image of Pi in the component group
// is ci mod m, where m is the exponent of the component group at p.
//
// Each ci is a vector of length 1 or 2 (the latter for when the
// component group is C2xC2), not just an integer.
//
// If p=0 then m=1 or 2 (m=2 iff there are two real components and at
// least one P_i is not in the connected component)
//
vector<vector<int> > MapPointsToComponentGroup(const CurveRed& CR, const vector<Point>& Plist, const bigint& p)
{
int i,j,k,n=Plist.size();
vector<vector<int> > images;
images.resize(n);
if (n==0) return images;
ComponentGroups CG(CR);
// Construct the component group and find its structure:
vector<int> G=CG.ComponentGroup(p);
#ifdef DEBUG_EGR
cout<<"Component group = "<<G<<endl;
#endif
int m = G.size();
int cyclic = (m==1);
int orderG = (cyclic? G[0]: 4);
// Initialize the image to 0:
for(i=0; i<n; i++)
{
images[i].resize(G.size());
for(j=0; j<m; j++) images[i][j]=0;
}
if (orderG==1) return images;
if (cyclic) // Now G is cyclic and nontrivial
{
#ifdef DEBUG_EGR
cout<< "cyclic case (order "<<orderG<<")"<<endl;
#endif
for(i=0; i<n; i++)
{
images[i][0]=CG.ImageInComponentGroup(Plist[i],p,G);
}
// if order =3 or =4, check for compatibility since our map is
// only then defined up to sign....
if((m==3)||(m==4))
{
// Find a point with image +1, if any:
int i0=-1;
for(i=0; i<n; i++) {if(images[i][0]==1) {i0=i;break;}}
if(i0!=-1) // else nothing to do
{
Point P0=Plist[i0];
for(i=i0+1; i<n; i++)
if(images[i][0]==1)
if(!CG.InSameComponent(P0,Plist[i],p))
images[i][0]=-1;
}
} // end of special treatment for m=3,4
} // end of cyclic case
else
{
#ifdef DEBUG_EGR
cout<< "non-cyclic case"<<endl;
#endif
// points representing up to 3 nontrivial components:
vector<Point> PointReps;
// the three nonzero images:
vector<vector<int> > ims(3);
ims[0]=vector<int>(2); ims[0][0]=1; ims[0][1]=0;
ims[1]=vector<int>(2); ims[1][0]=0; ims[1][1]=1;
ims[2]=vector<int>(2); ims[2][0]=1; ims[2][1]=1;
for (k=0; k<n; k++)
{
Point Pk=Plist[k];
#ifdef DEBUG_EGR
cout<< "processing point#"<<k<<": "<<Pk<<endl;
#endif
if(CG.HasGoodReduction(Pk,p)) continue;
int coset=-1;
for(unsigned int j=0; (j<PointReps.size())&&(coset==-1); j++)
if(CG.InSameComponent(Pk,PointReps[j],p))
coset=j;
if(coset==-1) // Pk is in a new coset...
{
#ifdef DEBUG_EGR
cout<<"Pk is in a new coset"<<endl;
#endif
coset=PointReps.size();
PointReps.push_back(Pk);
}
images[k]=ims[coset];
#ifdef DEBUG_EGR
cout<<"Pk is in coset #"<<(coset+1)<<", image = "<<images[k]<<endl;
#endif
} // loop on points
} // else ... noncyclic case
return images;
}
// returns m = the lcm of the exponents of the component groups at all
// bad primes (including infinity if real_too is 1), which is the lcm
// of the Tamagawa numbers (except: 2 when component group is of type
// 2,2). So with no further knowledge of the MW group we know that
// m*P is in the good-reduction subgroup for all P
bigint ComponentGroups::Tamagawa_exponent(int real_too)
{
const bigint one = BIGINT(1);
const bigint two = BIGINT(2);
bigint ans = one;
if(real_too && (conncomp==2)) ans = two;
map<bigint,Reduction_type>::const_iterator ri = reduct_array.begin();
for( ; ri!=reduct_array.end(); ri++)
{
int code=(ri->second).Kcode.code;
if((code%10==1)&&even((code-1)/10)) // Type I*m, m even: [2,2]
ans=lcm(ans,two);
else
ans=lcm(ans,BIGINT((ri->second).c_p));
}
return ans;
}
// class Kodaira_code just holds an int which "codes" the type as follows:
// (this coding originally from R.G.E.Pinch)
//
// Im -> 10*m
// I*m -> 10*m+1
// I, II, III, IV -> 1, 2, 3, 4
// I*, II*. III*, IV* -> 5, 6, 7, 8
//
//#define DEBUG_INDEX
bigint comp_map_image(const vector<int> moduli, const mat& image)
{
bigint ans; ans=1;
mat m=image;
#ifdef DEBUG_INDEX
cout<<"In comp_map_image, m="<<m;
cout<<"moduli = "<<moduli<<endl;
#endif
int npts=nrows(m), np=ncols(m);
int i, j, jj;
if(np==0) return ans;
for(j=1; j<=np; j++)
{
long modulus=moduli[j-1];
#ifdef DEBUG_INDEX
cout<<"Working on column "<<j<<", modulus "<<modulus<<endl;
#endif
if(modulus==1) continue;
#ifdef DEBUG_INDEX
cout<<"Column = "<<m.col(j)<<endl;
#endif
for(i=1; (i<=npts); i++) m(i,j)=m(i,j)%modulus;
long g=0,gm;
for(i=1; (i<=npts)&&(g!=1); i++) g=gcd(g,m(i,j));
#ifdef DEBUG_INDEX
cout<<"Column gcd = "<<g<<endl;
#endif
if(g==0) continue;
if(g>1)
{
gm=gcd(g,modulus);
if(gm>1)
{
modulus/=gm; g/=gm;
for(i=1; i<=npts; i++) m(i,j)=(m(i,j)/gm)%modulus;
}
if(g>1) for(i=1; i<=npts; i++) m(i,j)=(m(i,j)/g)%modulus;
}
#ifdef DEBUG_INDEX
cout<<"After scaling, modulus = "<<modulus<<endl;
#endif
if(modulus==1) continue;
#ifdef DEBUG_INDEX
cout<<"Column = "<<m.col(j)<<endl;
#endif
long colmin=modulus, imin=0, r, q;
while(abs(colmin)>1)
{
#ifdef DEBUG_INDEX
cout<<"colmin = "<<colmin<<endl;
#endif
for(i=1; i<=npts; i++)
if(m(i,j)!=0)
if(abs(m(i,j))<abs(colmin)) colmin=m(i,j);
for(i=1; i<=npts; i++) if(m(i,j)==colmin) {imin=i; break;}
#ifdef DEBUG_INDEX
cout<<"colmin = "<<colmin<<" at imin="<<imin<<endl;
#endif
for(i=1; i<=npts; i++)
if(ndivides(colmin,m(i,j)))
{
r = m(i,j) % colmin;
q = (m(i,j) - r) / colmin;
#ifdef DEBUG_INDEX
cout<<"subtracting "<<q<<" times row "<<imin<<" from row "<<i<<endl;
#endif
for(jj=1; jj<=np; jj++) m(i,jj)-=q*m(imin,jj);
}
#ifdef DEBUG_INDEX
cout<<"Column = "<<m.col(j)<<endl;
#endif
}
// now |colmin|=1 and we can clear
#ifdef DEBUG_INDEX
cout<<"colmin = "<<colmin<<", clearing..."<<endl;
cout<<"Column = "<<m.col(j)<<endl;
#endif
for(i=1; i<=npts; i++) if(m(i,j)==colmin) {imin=i; break;}
for(i=1; i<=npts; i++)
if(i!=imin)
for(jj=1; jj<=np; jj++)
{
if(colmin==1)
m(i,jj)-=m(i,j)*m(imin,jj);
else
m(i,jj)+=m(i,j)*m(imin,jj);
}
// now update matrix and ans:
#ifdef DEBUG_INDEX
cout<<"Column = "<<m.col(j)<<endl;
cout<<"multiplying ans and row "<<imin<<" by "<<modulus<<endl;
#endif
ans*=modulus;
for(jj=1; jj<=np; jj++)
m(imin,jj)=(modulus*m(imin,jj))%moduli[jj-1];
#ifdef DEBUG_INDEX
cout<<"Now matrix = "<<m<<endl;
cout<<"and ans = "<<ans<<endl;
#endif
}
return ans;
}
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