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Divisor :: dualize

dualize -- finds an ideal or module isomorphic to Hom(M, R)

Synopsis

Description

This computes $Hom_R(M, R)$.

i1 : R = QQ[x,y,z]/ideal(x^2-y*z);
 
i2 : m = ideal(x,y,z);

o2 : Ideal of R
 
i3 : dualize(m)

o3 = ideal x

o3 : Ideal of R
 
i4 : I = ideal(x,y);

o4 : Ideal of R
 
i5 : dualize(I)

o5 = ideal (z, x)

o5 : Ideal of R
 
i6 : dualize(I^2)

o6 = ideal z

o6 : Ideal of R
 
i7 : dualize(I^3)

             2
o7 = ideal (z , x*z)

o7 : Ideal of R

If Strategy => IdealStrategy, then dualize assume the module is isomorphic to an ideal, embeds it as an ideal, and computes the dual by forming a colon. ModuleStrategy simply computes the Hom. The default Strategy for modules is ModuleStrategy, and the default Strategy for ideals is IdealStrategy. This is because there is overhead using the opposite strategy (involving embedding modules as ideals). Frequently IdealStrategy is faster, but not always. Consider first a D4 singularity in characteristic 2.

i8 : R = ZZ/2[x,y,z]/ideal(z^2-x*y*z-x^2*y-x*y^2);
 
i9 : m = ideal(x,y,z);

o9 : Ideal of R
 
i10 : J = m^9;

o10 : Ideal of R
 
i11 : M = J*R^1;
 
i12 : time dualize(J, Strategy=>IdealStrategy);
     -- used 0.240044 seconds

o12 : Ideal of R
 
i13 : time dualize(J, Strategy=>ModuleStrategy);
     -- used 3.57903 seconds

o13 : Ideal of R
 
i14 : time dualize(M, Strategy=>IdealStrategy);
     -- used 3.81413 seconds
 
i15 : time dualize(M, Strategy=>ModuleStrategy);
     -- used 0.0138112 seconds
 
i16 : time embedAsIdeal dualize(M, Strategy=>ModuleStrategy);
     -- used 0.0110163 seconds

o16 : Ideal of R

For monomial ideals in toric rings, frequently ModuleStrategy appears faster.

i17 : R = ZZ/7[x,y,u,v]/ideal(x*y-u*v);
 
i18 : I = ideal(x,u);

o18 : Ideal of R
 
i19 : J = I^15;

o19 : Ideal of R
 
i20 : time dualize(J, Strategy=>IdealStrategy);
     -- used 0.319259 seconds

o20 : Ideal of R
 
i21 : time dualize(J, Strategy=>ModuleStrategy);
     -- used 0.0306731 seconds

o21 : Ideal of R

KnownDomain is an option for dualize. If it is false (default is true), then the computer will first check whether the ring is a domain, if it is not then it will revert to ModuleStrategy. If KnownDomain is set to true for a non-domain, then the function can return an incorrect answer.

i22 : R = QQ[x,y]/ideal(x*y);
 
i23 : J = ideal(x,y);

o23 : Ideal of R
 
i24 : dualize(J, KnownDomain=>true)

o24 = ideal x

o24 : Ideal of R
 
i25 : dualize(J, KnownDomain=>false)

o25 = ideal (y, x)

o25 : Ideal of R

See also

Ways to use dualize :

For the programmer

The object dualize is a method function with options.