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--- status: DRAFT
--- author(s): L.Gold
--- note:
document {
Key => hilbertSeries,
Headline => "compute the Hilbert series",
"The Hilbert series is the formal power series in the variables of the
degrees ring whose coefficients are the dimensions of the corresponding
graded component.",
"Note that the series is provided as a type of expression called
a ", TO "Divide", ".",
SeeAlso => {"degreesRing", "reduceHilbert", "poincare", "Complexes :: poincareN",
"hilbertPolynomial", "hilbertFunction"},
Subnodes => {
TO [hilbertSeries, Order],
TO [hilbertSeries, Reduce],
},
}
document {
Key => {(hilbertSeries, PolynomialRing), (hilbertSeries,QuotientRing)},
Headline => "compute the Hilbert series of a ring",
Usage => "hilbertSeries R",
Inputs => {
"R"
},
Outputs => {
Divide => "the Hilbert series" },
"We compute the ", TO2(hilbertSeries, "Hilbert series"), " of a ring.",
EXAMPLE {
"R = ZZ/101[x, Degrees => {2}]/ideal(x^2);",
"s = hilbertSeries R",
"numerator s",
"poincare R"
},
"Recall that the variables of the power series are the variables of
the ", TO2 (degreesRing,"degrees ring"), ".",
EXAMPLE {
"R=ZZ/101[x, Degrees => {{1,1}}]/ideal(x^2);",
"s = hilbertSeries R",
"numerator s",
"poincare R"
}
}
document {
Key => (hilbertSeries,Module),
Headline => "compute the Hilbert series of the module",
Usage => "hilbertSeries M",
Inputs => {
"M"
},
Outputs => {
Divide => "the Hilbert series"
},
"We compute the ", TO2 (hilbertSeries, "Hilbert series"), " of a
module.",
EXAMPLE {
"R = ZZ/101[x, Degrees => {2}];",
"M = module ideal x^2",
"s = hilbertSeries M",
"numerator s",
"poincare M"
},
"Recall that the variables of the power series are the variables of
the ", TO2 (degreesRing,"degrees ring"), ".",
EXAMPLE {
"R=ZZ/101[x, Degrees => {{1,1}}];",
"M = module ideal x^2;",
"s = hilbertSeries M",
"numerator s",
"poincare M"
}
}
document {
Key => {(hilbertSeries, Ideal)},
Headline => "compute the Hilbert series of the quotient of the ambient ring by the ideal",
Usage => "hilbertSeries I",
Inputs => {
"I" => Ideal
},
Outputs => {
Divide => "the Hilbert series" },
"We compute the ", TO2 (hilbertSeries, "Hilbert series"), " of ",
TT "R/I", ", the quotient of the ambient ring by the
ideal. Caution: For an ideal ", TT "I", " running ",
TT "hilbertSeries I ", "calculates the Hilbert series of ",
TT "R/I", ".",
EXAMPLE {
"R = ZZ/101[x, Degrees => {2}];",
"I = ideal x^2",
"s = hilbertSeries I",
"numerator s",
"poincare I",
"reduceHilbert s"
},
"Recall that the variables of the power series are the variables of
the ", TO2 (degreesRing,"degrees ring"), ".",
EXAMPLE {
"R=ZZ/101[x, Degrees => {{1,1}}];",
"I = ideal x^2;",
"s = hilbertSeries I",
"numerator s",
"poincare I",
"reduceHilbert s"
},
Caveat => {
"As is often the case, calling this function on an ideal ",
TT "I", " actually computes it for ", TT "R/I", " where ",
TT "R", " is the ring of ", TT "I", ".",
}
}
document {
Key => (hilbertSeries,ProjectiveHilbertPolynomial),
Headline => "compute the Hilbert series of a projective Hilbert polynomial",
Usage => "hilbertSeries P",
Inputs => {
"P" => ProjectiveHilbertPolynomial
},
Outputs => {
Divide => "the Hilbert series" },
"We compute the ", TO2 (hilbertSeries, "Hilbert series"), " of a
projective Hilbert polynomial.",
EXAMPLE {
"P = projectiveHilbertPolynomial 3",
"s = hilbertSeries P",
"numerator s"
},
"Computing the ", TO2 (hilbertSeries, "Hilbert series"), " of a
projective variety can be useful for finding the h-vector of a
simplicial complex from its f-vector. For example, consider the
octahedron. The ideal below is its Stanley-Reisner ideal. We can
see its f-vector (1, 6, 12, 8) in the Hilbert polynomial, and
then we get the h-vector (1,3,3,1) from the coefficients of the
Hilbert series projective Hilbert polynomial.",
EXAMPLE {
"R = QQ[a..h];",
"I = ideal (a*b, c*d, e*f);",
"P=hilbertPolynomial(I)",
"s = hilbertSeries P",
"numerator s"
}
}
document {
Key => [hilbertSeries, Order],
Headline => "display the truncated power series expansion",
Usage => "hilbertSeries(..., Order => n)",
Inputs => {
"n" => ZZ},
Consequences => {
{"The output is no longer of type ", TO "Divide", ". It is a
polynomial in the ", TO2 (degreesRing,"degrees ring"), "."}
},
"We compute the Hilbert series both without and with the optional
argument. In the second case notice the terms of power series
expansion up to, but not including, degree 5 are displayed rather
than expressing the series as a rational function. The polynomial
expression is an element of a Laurent polynomial ring that is
the ", TO2 (degreesRing,"degrees ring"), " of the ambient ring.",
EXAMPLE {
"R = ZZ/101[x,y];",
"hilbertSeries(R/x^3)",
"hilbertSeries(R/x^3, Order =>5)"
},
"If the ambient ring is multigraded, then the ",
TO2 (degreesRing,"degrees ring"), " has multiple variables.",
EXAMPLE {
"R = ZZ/101[x,y, Degrees=>{{1,2},{2,3}}];",
"hilbertSeries(R/x^3, Order =>5)"
},
"The heft vector provides a suitable monomial ordering and degrees in the ring of the Hilbert series.",
EXAMPLE lines ///
R = QQ[a..d,Degrees=>{{-2,-1},{-1,0},{0,1},{1,2}}]
hilbertSeries(R, Order =>3)
degrees ring oo
heft R
///,
}
document {
Key => [hilbertSeries,Reduce],
Headline => "reduce the Hilbert series",
Usage => "hilbertSeries(..., Reduce => true)",
Consequences => {{"the resulting rational function is reduced by cancelling
factors of the numerator that occur explicitly as factors of the denominator.
See also ", TO "reduceHilbert", "."
}},
EXAMPLE lines ///
R = QQ[x,y,z];
hilbertSeries ideal (x,y)
hilbertSeries(ideal (x,y), Reduce => true)
///
}
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