Macaulay2 » Documentation
Packages » TestIdeals :: isFPure
next | previous | forward | backward | up | index | toc

isFPure -- whether a ring is F-pure



Given a ring $R$, this function checks whether the ring is $F$-pure, using Fedder's criterion (by applying frobeniusRoot to $I^{[p]} : I$, where $I$ is the defining ideal of $R$).

i1 : R = ZZ/5[x,y,z]/(x^2 + y*z);
i2 : isFPure(R)

o2 = true
i3 : R = ZZ/7[x,y,z]/(x^3 + y^3 + z^3);
i4 : isFPure(R)

o4 = true
i5 : R = ZZ/5[x,y,z]/(x^3 + y^3 + z^3);
i6 : isFPure(R)

o6 = false

Alternatively, one may pass the defining ideal of a ring.

i7 : S = ZZ/2[x,y,z];
i8 : isFPure ideal(y^2 - x^3)

o8 = false
i9 : isFPure ideal(z^2 - x*y*z + x*y^2 + x^2*y)

o9 = true

The option AtOrigin controls whether $F$-purity is checked at the origin or everywhere. If its value is set to true (the default is false), it will only check $F$-purity at the origin.

i10 : R = ZZ/5[x,y,z]/((x - 1)^3 + (y - 2)^3 + z^3);
i11 : isFPure(R)

o11 = false
i12 : isFPure(R, AtOrigin => true)

o12 = true
i13 : S = ZZ/13[x,y,z]/(x^3 + y^3 + z^3);
i14 : isFPure(S)

o14 = true
i15 : isFPure(S, AtOrigin => true)

o15 = true

Note that there is a difference in the strategy for the local or non-local computations. In fact, checking $F$-purity globally can sometimes be faster than checking it only at the origin. If AtOrigin is set to false, then isFPure computes the frobeniusRoot of $I^{[p]} : I$, whereas if AtOrigin is set to true, it checks whether $I^{[p]} : I$ is contained in $m^{[p]}$, where $m$ is the maximal ideal generated by the variables.

The option FrobeniusRootStrategy is passed to internal frobeniusRoot calls, and specifies the strategy to be used.

See also

Ways to use isFPure :

For the programmer

The object isFPure is a method function with options.