# Difference between revisions of "ApCoCoA-1:NCo.HF"

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<em>Please note:</em> The function(s) explained on this page is/are using the <em>ApCoCoAServer</em>. You will have to start the ApCoCoAServer in order to use it/them. | <em>Please note:</em> The function(s) explained on this page is/are using the <em>ApCoCoAServer</em>. You will have to start the ApCoCoAServer in order to use it/them. | ||

<par/> | <par/> | ||

− | Please set ring environment <em>coefficient field</em> <tt> K</tt>, <em>alphabet</em> (or set of indeterminates) <tt>X</tt> and <em>ordering</em> via the functions <ref>ApCoCoA-1:NCo.SetFp|NCo.SetFp</ref>, <ref>ApCoCoA-1:NCo.SetX|NCo.SetX</ref> and <ref>ApCoCoA-1:NCo.SetOrdering|NCo.SetOrdering</ref>, respectively, before calling this function. The default coefficient field is <tt>Q</tt>, and the default ordering is the length-lexicographic ordering ( | + | Please set ring environment <em>coefficient field</em> <tt> K</tt>, <em>alphabet</em> (or set of indeterminates) <tt>X</tt> and <em>ordering</em> via the functions <ref>ApCoCoA-1:NCo.SetFp|NCo.SetFp</ref>, <ref>ApCoCoA-1:NCo.SetX|NCo.SetX</ref> and <ref>ApCoCoA-1:NCo.SetOrdering|NCo.SetOrdering</ref>, respectively, before calling this function. The default coefficient field is <tt>Q</tt>, and the default ordering is the length-lexicographic ordering ("LLEX"). For more information, please check the relevant functions. |

<itemize> | <itemize> | ||

− | <item>@param <em>Gb:</em> a LIST of non-zero polynomials in <tt>K<X></tt> forming a Groebner basis with respect to a length compatible word ordering. Each polynomial is represented as a LIST of monomials, which are LISTs of the form [C, W] where W is a word in <tt><X></tt> and C is the coefficient of W. For example, the polynomial <tt>f=xy-y+1</tt> is represented as F:=[[1, | + | <item>@param <em>Gb:</em> a LIST of non-zero polynomials in <tt>K<X></tt> forming a Groebner basis with respect to a length compatible word ordering. Each polynomial is represented as a LIST of monomials, which are LISTs of the form [C, W] where W is a word in <tt><X></tt> and C is the coefficient of W. For example, the polynomial <tt>f=xy-y+1</tt> is represented as F:=[[1,"xy"], [-1, "y"], [1,""]]. <em>Warning:</em> users should take responsibility to make sure that Gb is indeed a Groebner basis with respect to a length compatible word ordering! In the case that Gb is a partical Groebner basis, the function enumerates the values of a pseudo Hilbert function.</item> |

<item>@return: a LIST of non-negative integers, which are values of the Hilbert function of the <tt>K</tt>-algebra <tt>K<X>/<Gb></tt>.</item> | <item>@return: a LIST of non-negative integers, which are values of the Hilbert function of the <tt>K</tt>-algebra <tt>K<X>/<Gb></tt>.</item> | ||

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</itemize> | </itemize> | ||

<example> | <example> | ||

− | NCo.SetX( | + | NCo.SetX("xyzt"); |

− | NCo.SetOrdering( | + | NCo.SetOrdering("LLEX"); |

− | Gb:= [[[1, | + | Gb:= [[[1, "yt"], [-1, "ty"]], [[1, "xt"], [-1, "tx"]], [[1, "xy"], [-1, "ty"]], [[1, "xx"], [-1, "yx"]], |

− | [[1, | + | [[1, "tyy"], [-1, "tty"]], [[1, "yyx"], [-1, "tyx"]]]; |

NCo.HF(Gb, 5); | NCo.HF(Gb, 5); | ||

[1, 4, 12, 34, 100, 292] | [1, 4, 12, 34, 100, 292] |

## Latest revision as of 13:39, 29 October 2020

This article is about a function from ApCoCoA-1. |

## NCo.HF

Enumerate values of the Hilbert-Dehn function of a finitely generated `K`-algebra.

### Syntax

NCo.HF(Gb:LIST[, DB:INT]):LIST

### Description

Let `K<X>` be a finitely generated non-commutative polynomial ring, and let `I` be a finitely generated two-sided ideal in `K<X>`. Then `K<X>/I` is a finitely generated `I`-algebra. For every integer `i` in `N`, we let `F_{i}` be the `K`-vector subspace generated by the words of length less than or equal to `i`. Then `{F_{i}}` is a filtration of `K<X>`. Further, the filtration `{F_{i}}` induces a filtration `{F_{i}/(F_{i} intersects I)}` of `K<X>/I`. The *Hilbert function* of `K<X>/I` is a map `HF: N -->` N defined by `HF(i)=dim(F_{i}/(F_{i} intersects I))-dim(F_{i-1}/(F_{i-1} intersects I))`, i.e. `HF(i)` is equal to the number of words of length `i` in a Macaulay's basis (see NCo.MB) of `K<X>/I`.

*Please note:* The function(s) explained on this page is/are using the *ApCoCoAServer*. You will have to start the ApCoCoAServer in order to use it/them.

Please set ring environment *coefficient field* ` K`, *alphabet* (or set of indeterminates) `X` and *ordering* via the functions NCo.SetFp, NCo.SetX and NCo.SetOrdering, respectively, before calling this function. The default coefficient field is `Q`, and the default ordering is the length-lexicographic ordering ("LLEX"). For more information, please check the relevant functions.

@param

*Gb:*a LIST of non-zero polynomials in`K<X>`forming a Groebner basis with respect to a length compatible word ordering. Each polynomial is represented as a LIST of monomials, which are LISTs of the form [C, W] where W is a word in`<X>`and C is the coefficient of W. For example, the polynomial`f=xy-y+1`is represented as F:=[[1,"xy"], [-1, "y"], [1,""]].*Warning:*users should take responsibility to make sure that Gb is indeed a Groebner basis with respect to a length compatible word ordering! In the case that Gb is a partical Groebner basis, the function enumerates the values of a pseudo Hilbert function.@return: a LIST of non-negative integers, which are values of the Hilbert function of the

`K`-algebra`K<X>/<Gb>`.

Optional parameter:

@param

*DB:*a positive INT, which is a degree bound of the Hilbert function.*Note that*we set DB=32 by default. Thus, in the case that the`K`-dimension of`K<X>/<Gb>`is finite, it is necessary to set DB to a large enough INT in order to compute all the values of the Hilbert function.

#### Example

NCo.SetX("xyzt"); NCo.SetOrdering("LLEX"); Gb:= [[[1, "yt"], [-1, "ty"]], [[1, "xt"], [-1, "tx"]], [[1, "xy"], [-1, "ty"]], [[1, "xx"], [-1, "yx"]], [[1, "tyy"], [-1, "tty"]], [[1, "yyx"], [-1, "tyx"]]]; NCo.HF(Gb, 5); [1, 4, 12, 34, 100, 292] -------------------------------

### See also