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A.21 Primary decomposition

There are two algorithms implemented in SINGULAR which provide primary decomposition: primdecGTZ, based on Gianni/Trager/Zacharias (written by Gerhard Pfister) and primdecSY, based on Shimoyama/Yokoyama (written by Wolfram Decker and Hans Schoenemann).

The result of primdecGTZ and primdecSY is returned as a list of pairs of ideals, where the second ideal form the prime ideal and the first ideal form the corresponding primary ideal.

 
  LIB "primdec.lib";
  ring r = 0,(a,b,c,d,e,f),dp;
  ideal i= f3, ef2, e2f, bcf-adf, de+cf, be+af, e3;
  primdecGTZ(i);
→ [1]:
→    [1]:
→       _[1]=f
→       _[2]=e
→    [2]:
→       _[1]=f
→       _[2]=e
→ [2]:
→    [1]:
→       _[1]=f3
→       _[2]=ef2
→       _[3]=e2f
→       _[4]=e3
→       _[5]=de+cf
→       _[6]=be+af
→       _[7]=-bc+ad
→    [2]:
→       _[1]=f
→       _[2]=e
→       _[3]=-bc+ad
  // We consider now the ideal J of the base space of the
  // miniversal deformation of the cone over the rational
  // normal curve computed in section *8* and compute
  // its primary decomposition.
  ring R = 0,(A,B,C,D),dp;
  ideal J = CD, BD+D2, AD;
  primdecGTZ(J);
→ [1]:
→    [1]:
→       _[1]=D
→    [2]:
→       _[1]=D
→ [2]:
→    [1]:
→       _[1]=C
→       _[2]=B+D
→       _[3]=A
→    [2]:
→       _[1]=C
→       _[2]=B+D
→       _[3]=A
  // We see that there are two components which are both
  // prime, even linear subspaces, one 3-dimensional,
  // the other 1-dimensional.
  // (This is Pinkhams example and was the first known
  // surface singularity with two components of
  // different dimensions)
  //
  // Let us now produce an embedded component in the last
  // example, compute the minimal associated primes and
  // the radical. We use the Characteristic set methods
  // from prim_dec.lib.
  J = intersect(J,maxideal(3));
  // The following shows that the maximal ideal defines an embedded
  // (prime) component.
  primdecSY(J);
→ [1]:
→    [1]:
→       _[1]=D
→    [2]:
→       _[1]=D
→ [2]:
→    [1]:
→       _[1]=C
→       _[2]=B+D
→       _[3]=A
→    [2]:
→       _[1]=C
→       _[2]=B+D
→       _[3]=A
→ [3]:
→    [1]:
→       _[1]=D2
→       _[2]=C2
→       _[3]=B2
→       _[4]=AB
→       _[5]=A2
→       _[6]=BCD
→       _[7]=ACD
→    [2]:
→       _[1]=D
→       _[2]=C
→       _[3]=B
→       _[4]=A
  minAssChar(J);
→ [1]:
→    _[1]=C
→    _[2]=B+D
→    _[3]=A
→ [2]:
→    _[1]=D
  radical(J);
→ _[1]=CD
→ _[2]=BD+D2
→ _[3]=AD

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