#### D.10.1.1 Adj_div

Procedure from library `brnoeth.lib` (see brnoeth_lib).

Usage:
  Adj_div( f [,l] ); f a poly, [l a list]

Return:
  list L with the computed data:

        L[1] a list of rings: L[1][1]=aff_r (affine), L[1][2]=Proj_R (projective),
        L[2] an intvec with 2 entries (degree, genus),
        L[3] a list of intvec (closed places),
        L[4] an intvec (conductor),
        L[5] a list of lists:
           L[5][d][1] a (local) ring over an extension of degree d,
           L[5][d][2] an intvec (degrees of base points of places of degree d)

Note:
  `Adj_div(f);` computes and stores the fundamental data of the plane curve defined by f as needed for AG codes.
  In the affine ring you can find the following data:

         poly CHI:  affine equation of the curve,
         ideal Aff_SLocus:  affine singular locus (std),
         list Inf_Points:  points at infinity
                  Inf_Points[1]:  singular points
                  Inf_Points[2]:  non-singular points,
         list Aff_SPoints:  affine singular points (if not empty).

  In the projective ring you can find the projective equation CHI of the curve (poly).
  In the local rings L[5][d][1] you find:

         list POINTS:  base points of the places of degree d,
         list LOC_EQS:  local equations of the curve at the base points,
         list BRANCHES:  Hamburger-Noether developments of the places,
         list PARAMETRIZATIONS:  local parametrizations of the places,

  Each entry of the list L[3] corresponds to one closed place (i.e., a place and all its conjugates) which is represented by an intvec of size two, the first entry is the degree of the place (in particular, it tells the local ring where to find the data describing one representative of the closed place), and the second one is the position of those data in the lists POINTS, etc., inside this local ring.
  In the intvec L[4] (conductor) the i-th entry corresponds to the i-th entry in the list of places L[3].

  With no optional arguments, the conductor is computed by local invariants of the singularities; otherwise it is computed by the Dedekind formula.
  An affine point is represented by a list P where P[1] is std of a prime ideal and P[2] is an intvec containing the position of the places above P in the list of closed places L[3].
  If the point is at infinity, P[1] is a homogeneous irreducible polynomial in two variables.

  If `printlevel>=0` additional comments are displayed (default: `printlevel=0`).

Warning:
  The parameter of the needed field extensions is called 'a'. Thus, there should be no global object named 'a' when executing Adj_div.

Example:

    LIB "brnoeth.lib";
    int plevel=printlevel;
    printlevel=-1;
    ring s=2,(x,y),lp;
    list C=Adj_div(y9+y8+xy6+x2y3+y2+x3);
    ==> The genus of the curve is 3
    def aff_R=C[1][1];      // the affine ring
    setring aff_R;
    listvar(aff_R);         // data in the affine ring
    ==> // aff_R                          [0]  *ring
    ==> // Inf_Points                     [0]  list, size: 2
    ==> // Aff_SPoints                    [0]  list, size: 3
    ==> // Aff_SLocus                     [0]  ideal (SB), 2 generator(s)
    ==> // CHI                            [0]  poly
    CHI;                    // affine equation of the curve
    ==> x3+x2y3+xy6+y9+y8+y2
    Aff_SLocus;             // ideal of the affine singular locus
    ==> Aff_SLocus[1]=y8+y2
    ==> Aff_SLocus[2]=x2+y6
    Aff_SPoints[1];         // 1st affine singular point: (1:1:1), no.1
    ==> [1]:
    ==>    _[1]=y
    ==>    _[2]=x
    ==> [2]:
    ==>    1
    Inf_Points[1];          // singular point(s) at infinity: (1:0:0), no.4
    ==> [1]:
    ==>    [1]:
    ==>       y
    ==>    [2]:
    ==>       4
    Inf_Points[2];          // list of non-singular points at infinity
    ==> empty list
    //
    def proj_R=C[1][2];     // the projective ring
    setring proj_R;
    CHI;                    // projective equation of the curve
    ==> x3z6+x2y3z4+xy6z2+y9+y8z+y2z7
    C[2][1];                // degree of the curve
    ==> 9
    C[2][2];                // genus of the curve
    ==> 3
    C[3];                   // list of computed places
    ==> [1]:
    ==>    1,1
    ==> [2]:
    ==>    1,2
    ==> [3]:
    ==>    2,1
    ==> [4]:
    ==>    1,3
    C[4];                   // adjunction divisor (all points are singular!)
    ==> 2,2,2,42
    //
    // we look at the place(s) of degree 2 by changing to the ring
    C[5][2][1];
    ==> // coefficients: ZZ/2[a]/(...) considered as a field
    ==> // number of vars : 3
    ==> //        block   1 : ordering ls
    ==> //                  : names    x y t
    ==> //        block   2 : ordering C
    def S(2)=C[5][2][1];
    setring S(2);
    POINTS;                // base point(s) of place(s) of degree 2: (1:a:1)
    ==> [1]:
    ==>    [1]:
    ==> 1
    ==>    [2]:
    ==> (a)
    ==>    [3]:
    ==> 1
    LOC_EQS;               // local equation(s)
    ==> [1]:
    ==>    y2+y3+(a+1)*y4+y6+(a+1)*y8+y9+(a)*xy2+(a+1)*xy4+xy6+(a+1)*x2y+(a)*x2y2\
       +x2y3+x3
    PARAMETRIZATIONS;      // parametrization(s) and exactness
    ==> [1]:
    ==>    [1]:
    ==>       _[1]=t2+(a+1)*t3
    ==>       _[2]=t3+(a+1)*t4
    ==>    [2]:
    ==>       3,4
    BRANCHES;              // Hamburger-Noether development
    ==> [1]:
    ==>    [1]:
    ==>       _[1,1]=0
    ==>       _[1,2]=x
    ==>       _[1,3]=0
    ==>       _[2,1]=0
    ==>       _[2,2]=1
    ==>       _[2,3]=(a+1)
    ==>    [2]:
    ==>       1,-4
    ==>    [3]:
    ==>       0
    ==>    [4]:
    ==>       y+(a+1)*xy+(a)*x2y+(a)*x2y2+(a+1)*x3+x3y+x3y3+(a)*x4+(a+1)*x4y2+(a+\
       1)*x4y3+x5+x5y2+(a)*x6+(a+1)*x6y2+x6y4+x6y5+x7y+(a+1)*x8+(a+1)*x8y+x8y4+(\
       a+1)*x8y6+x9+x9y7+(a+1)*x10+x11y6+(a+1)*x12y4+x13y5+x14+x14y+x15y4+x16+(a\
       +1)*x16y2+x17y3+x19y2+(a+1)*x20+x21y+x23
    printlevel=plevel;

See also: NSplaces; closed_points.

User manual for Singular version 4.4.1, 2025, generated by texi2html.