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document {
Key => Ext,
Headline => "compute an Ext module"
}
document {
Key => {(Ext,Module,Module),(Ext,Ideal,Ideal),(Ext,Ideal,Module),(Ext,Ideal,Ring),(Ext,Module,Ideal),(Ext,Module,Ring)},
Headline => "total Ext module",
Usage => "Ext(M,N)",
Inputs => { "M" => {ofClass{Ideal,Ring}}, "N" => {ofClass{Ideal,Ring}}},
Outputs => {
TEX { "the $Ext$ module of $M$ and $N$,
as a multigraded module, with the modules $Ext^i(M,N)$ for all values of $i$ appearing simultaneously." }},
PARA { "The modules ", TT "M", " and ", TT "N", " should be graded (homogeneous) modules over the same ring." },
PARA { "If ", TT "M", " or ", TT "N", " is an ideal or ring, it is regarded as a module in the evident way." },
PARA TEX {
"The computation of the total Ext module is possible for modules over the
ring $R$ of a complete intersection, according the algorithm
of Shamash-Eisenbud-Avramov-Buchweitz. The result is provided as a finitely
presented module over a new ring with one additional variable of degree
", TT "{-2,-d}", " for each equation of degree ", TT "d", " defining $R$. The
variables in this new ring have degree length 1 more than the degree length of
the original ring, i.e., is multigraded, with the
degree ", TT "d", " part of $Ext^n(M,N)$ appearing as the degree
", TT "prepend(-n,d)", " part of ", TT "Ext(M,N)", ". We illustrate this in
the following example."
},
EXAMPLE lines ///
R = QQ[x,y]/(x^3,y^2);
N = cokernel matrix {{x^2, x*y}}
H = Ext(N,N);
ring H
S = ring H;
H
isHomogeneous H
rank source basis( { -2,-3 }, H)
rank source basis( { -3 }, Ext^2(N,N) )
rank source basis( { -4,-5 }, H)
rank source basis( { -5 }, Ext^4(N,N) )
hilbertSeries H
hilbertSeries(H,Order=>11)
///,
PARA{ "The result of the computation is cached for future reference." }
}
document {
Key => {(Ext,ZZ,Module,Module),
(Ext, ZZ, Ideal, Ideal),
(Ext, ZZ, Ideal, Module),
(Ext, ZZ, Module, Ideal),
(Ext, ZZ, Ideal, Ring),
(Ext, ZZ, Module, Ring)
},
Usage => "Ext^i(M,N)",
Headline => "Ext module",
Inputs => { "i", "M", "N" },
Outputs => {
{ "the ", TT "i", "-th ", TT "Ext", " module of ", TT "M", " and ", TT "N" }
},
"If ", TT "M", " or ", TT "N", " is an ideal or ring, it is regarded as a module in the evident way.",
EXAMPLE lines ///
R = ZZ/32003[a..d];
I = monomialCurveIdeal(R,{1,3,4})
M = R^1/I
Ext^1(M,R)
Ext^2(M,R)
Ext^3(M,R)
Ext^1(I,R)
///,
"As an efficiency consideration, it is generally much more efficient to compute
Ext^i(R^1/I,N) rather than Ext^(i-1)(I,N). The latter first computes a presentation
of the ideal I, and then a free resolution of that. For many examples, the
difference in time and space required can be very large.",
SeeAlso => {resolution,Tor,Hom,monomialCurveIdeal,(Ext,ZZ,Matrix,Module),(Ext,ZZ,Module,Matrix)}
}
document {
Key => {(Ext, ZZ, Matrix, Module),
(Ext, ZZ, Matrix, Ideal),
(Ext, ZZ, Matrix, Ring)
},
Usage => "Ext^i(f,N)",
Headline => "map between Ext modules",
Inputs => { "i", "f" => "M1 --> M2", "N" },
Outputs => {
Matrix => {TEX ///the map $Ext^i(M2,N) \rightarrow{} Ext^i(M1,N)$///}
},
"If ", TT "N", " is an ideal or ring, it is regarded as a module in the evident way.",
EXAMPLE lines ///
R = ZZ/32003[a..d];
I = monomialCurveIdeal(R,{1,3,4})
M1 = R^1/I
M2 = R^1/ideal(I_0,I_1)
f = inducedMap(M1,M2)
Ext^1(f,R)
g = Ext^2(f,R)
source g == Ext^2(M1,R)
target g == Ext^2(M2,R)
Ext^3(f,R)
///,
SeeAlso => {resolution,Tor,Hom,(Ext,ZZ,Module,Module)}
}
document {
Key => {(Ext, ZZ, Module, Matrix),
(Ext, ZZ, Ideal, Matrix)
},
Usage => "Ext^i(M,f)",
Headline => "map between Ext modules",
Inputs => { "i", "M", "f" => "N1 --> N2"},
Outputs => {
Matrix => {TEX ///the induced map $Ext^i(M,N1) \rightarrow{} Ext^i(M,N2)$///}
},
"If ", TT "M", " is an ideal, it is regarded as a module in the evident way.",
PARA{},
-- the code for Hom(Module,Matrix) is wrong, so we disable this example temporarily
-- EXAMPLE lines ///
-- R = ZZ/32003[a..d];
-- I = monomialCurveIdeal(R,{1,3,4})
-- M = R^1/I
-- f = map(R^1,module I,gens I)
-- Ext^1(M,f)
-- g = Ext^2(M,f)
-- source g == Ext^2(M,source f)
-- target g == Ext^2(M,target f)
-- Ext^3(f,R)
-- ///,
SeeAlso => {resolution,Tor,Hom,(Ext,ZZ,Module,Module),(Ext,ZZ,Matrix,Module)}
}
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