### A.4.5 Deformations

- The libraries `sing.lib`, respextively `deform.lib`, contain procedures to compute total and base space of the miniversal (= semiuniversal) deformation of an isolated complete intersection singularity, respectively of an arbitrary isolated singularity.
- The procedure `deform` in `sing.lib` returns a matrix whose columns represent all 1st order deformations. More precisely, if is the ideal generated by , then any infinitesimal deformation of over is given by , where , and where is a -linear combination of the .
- The procedure `versal` in `deform.lib` computes a formal miniversal deformation up to a certain order which can be prescribed by the user. For a complete intersection the 1st order part is already miniversal.
- The procedure `versal` extends the basering to a new ring with additional deformation parameters which contains the equations for the miniversal base space and the miniversal total space.
- There are default names for the objects created, but the user may also choose their own names.
- If the user sets `printlevel=2;` before running `versal`, some intermediate results are shown. This is useful since `versal` is already complicated and might run for some time on more complicated examples. (type `help versal;`)

We compute for the same examples as in the section T1 and T2 the miniversal deformations:

      LIB "deform.lib";
      ring R=32003,(x,y,z),ds;
      //----------------------------------------------------
      // hypersurface case (from series T[p,q,r]):
      int p,q,r = 3,3,4;
      poly f = x^p+y^q+z^r+xyz;
      print(deform(f));
    ==> z3,z2,yz,xz,z,y,x,1
      // the miniversal deformation of f=0 is the projection from the
      // miniversal total space to the miniversal base space:
      // { (A,B,C,D,E,F,G,H,x,y,z) | x3+y3+xyz+z4+A+Bx+Cxz+Dy+Eyz+Fz+Gz2+Hz3 =0 }
      //  --> { (A,B,C,D,E,F,G,H) }
      //----------------------------------------------------
      // complete intersection case (from series P[k,l]):
      int k,l =3,2;
      ideal j=xy,x^k+y^l+z2;
      print(deform(j));
    ==> 0,0, 0,0,z,1,
    ==> y,x2,x,1,0,0
      def L=versal(j);            // using default names
    ==> // smooth base space
    ==> // ready: T_1 and T_2
    ==>
    ==>
    ==> // 'versal' returned a list, say L, of four rings. In L[1] are stored:
    ==> //   as matrix Fs: Equations of total space of the miniversal deformation\
       ,
    ==> //   as matrix Js: Equations of miniversal base space,
    ==> //   as matrix Rs: syzygies of Fs mod Js.
    ==> // To access these data, type
    ==>      def Px=L[1]; setring Px; print(Fs); print(Js); print(Rs);
    ==>
    ==> // L[2] = L[1]/Fo extending Qo=Po/Fo,
    ==> // L[3] = the embedding ring of the versal base space,
    ==> // L[4] = L[1]/Js extending L[3]/Js.
    ==>
      def Px=L[1]; setring Px;
      show(Px);                   // show is a procedure from inout.lib
    ==> // ring: (32003),(A,B,C,D,E,F,x,y,z),(ds(6),ds(3),C);
    ==> // minpoly = 0
    ==> // objects belonging to this ring:
    ==> // Rs                             [0]  matrix 2 x 1
    ==> // Fs                             [0]  matrix 1 x 2
    ==> // Js                             [0]  matrix 1 x 0
      listvar(matrix);
    ==> // Rs                             [0]  matrix 2 x 1
    ==> // Fs                             [0]  matrix 1 x 2
    ==> // Js                             [0]  matrix 1 x 0
      // ___ Equations of miniversal base space ___:
      Js;
    ==>
      // ___ Equations of miniversal total space ___:
      Fs;
    ==> Fs[1,1]=xy+Ez+F
    ==> Fs[1,2]=y2+z2+x3+Ay+Bx2+Cx+D
      // the miniversal deformation of V(j) is the projection from the
      // miniversal total space to the miniversal base space:
      // { (A,B,C,D,E,F,x,y,z) | xy+F+Ez=0, y2+z2+x3+D+Cx+Bx2+Ay=0 }
      //  --> { (A,B,C,D,E,F) }
      //----------------------------------------------------
      // general case (cone over rational normal curve of degree 4):
      kill L;
      ring r1=0,(x,y,z,u,v),ds;
      matrix m[2][4]=x,y,z,u,y,z,u,v;
      ideal i=minor(m,2);                 // 2x2 minors of matrix m
      int time=timer;
      // Call parameters of the miniversal base A(1),A(2),...:
      def L=versal(i,0,"","A(");
    ==> // ready: T_1 and T_2
    ==> // start computation in degree 2.
    ==> // ** J is no standard basis
    ==>
    ==>
    ==> // 'versal' returned a list, say L, of four rings. In L[1] are stored:
    ==> //   as matrix Fs: Equations of total space of the miniversal deformation\
       ,
    ==> //   as matrix Js: Equations of miniversal base space,
    ==> //   as matrix Rs: syzygies of Fs mod Js.
    ==> // To access these data, type
    ==>      def Px=L[1]; setring Px; print(Fs); print(Js); print(Rs);
    ==>
    ==> // L[2] = L[1]/Fo extending Qo=Po/Fo,
    ==> // L[3] = the embedding ring of the versal base space,
    ==> // L[4] = L[1]/Js extending L[3]/Js.
    ==>
      "// used time:",timer-time,"sec";   // time of last command
    ==> // used time: 0 sec
      def Def_rPx=L[1]; setring Def_rPx;
      Fs;
    ==> Fs[1,1]=-u^2+z*v+A(2)*u+A(4)*v
    ==> Fs[1,2]=-z*u+y*v-A(1)*u+A(4)*u
    ==> Fs[1,3]=-y*u+x*v+A(3)*u+A(4)*z
    ==> Fs[1,4]=z^2-y*u+A(1)*z+A(2)*y
    ==> Fs[1,5]=y*z-x*u+A(2)*x-A(3)*z
    ==> Fs[1,6]=-y^2+x*z+A(1)*x+A(3)*y
      Js;
    ==> Js[1,1]=A(2)*A(4)
    ==> Js[1,2]=A(1)*A(4)-A(4)^2
    ==> Js[1,3]=-A(3)*A(4)
      // the miniversal deformation of V(i) is the projection from the
      // miniversal total space to the miniversal base space:
      // { (A(1..4),x,y,z,u,v) |
      //         -u^2+x*v+A(2)*u+A(4)*v=0, -z*u+y*v-A(1)*u+A(3)*u=0,
      //         -y*u+x*v+A(3)*u+A(4)*z=0,  z^2-y*u+A(1)*z+A(2)*y=0,
      //          y*z-x*u+A(2)*x-A(3)*z=0, -y^2+x*z+A(1)*x+A(3)*y=0 }
      //  --> { A(1..4) |
      //          A(2)*A(4) = -A(3)*A(4) = -A(1)*A(4)+A(4)^2 = 0 }
      //----------------------------------------------------

User manual for Singular version 4-0-3, 2016, generated by texi2html.