[1X4 [33X[0;0YPcp-groups - polycyclically presented groups[133X[101X [1X4.1 [33X[0;0YPcp-elements -- elements of a pc-presented group[133X[101X [33X[0;0YA [13Xpcp-element[113X is an element of a group defined by a consistent pc-presentation given by a collector. Suppose that [22Xg_1, ..., g_n[122X are the defining generators of the collector. Recall that each element [22Xg[122X in this group can be written uniquely as a collected word [22Xg_1^e_1 ⋯ g_n^e_n[122X with [22Xe_i ∈ [122ℤX and [22X0 ≤ e_i < r_i[122X for [22Xi ∈ I[122X. The integer vector [22X[e_1, ..., e_n][122X is called the [13Xexponent vector[113X of [22Xg[122X. The following functions can be used to define pcp-elements via their exponent vector or via an arbitrary generator exponent word as introduced in Chapter [14X3[114X.[133X [1X4.1-1 PcpElementByExponentsNC[101X [33X[1;0Y[29X[2XPcpElementByExponentsNC[102X( [3Xcoll[103X, [3Xexp[103X ) [32X function[133X [33X[1;0Y[29X[2XPcpElementByExponents[102X( [3Xcoll[103X, [3Xexp[103X ) [32X function[133X [33X[0;0Yreturns the pcp-element with exponent vector [3Xexp[103X. The exponent vector is considered relative to the defining generators of the pc-presentation.[133X [1X4.1-2 PcpElementByGenExpListNC[101X [33X[1;0Y[29X[2XPcpElementByGenExpListNC[102X( [3Xcoll[103X, [3Xword[103X ) [32X function[133X [33X[1;0Y[29X[2XPcpElementByGenExpList[102X( [3Xcoll[103X, [3Xword[103X ) [32X function[133X [33X[0;0Yreturns the pcp-element with generators exponent list [3Xword[103X. This list [3Xword[103X consists of a sequence of generator numbers and their corresponding exponents and is of the form [22X[i_1, e_i_1, i_2, e_i_2, ..., i_r, e_i_r][122X. The generators exponent list is considered relative to the defining generators of the pc-presentation.[133X [33X[0;0YThese functions return pcp-elements in the category [10XIsPcpElement[110X. Presently, the only representation implemented for this category is [10XIsPcpElementRep[110X. (This allows us to be a little sloppy right now. The basic set of operations for [10XIsPcpElement[110X has not been defined yet. This is going to happen in one of the next version, certainly as soon as the need for different representations arises.)[133X [1X4.1-3 IsPcpElement[101X [33X[1;0Y[29X[2XIsPcpElement[102X( [3Xobj[103X ) [32X Category[133X [33X[0;0Yreturns true if the object [3Xobj[103X is a pcp-element.[133X [1X4.1-4 IsPcpElementCollection[101X [33X[1;0Y[29X[2XIsPcpElementCollection[102X( [3Xobj[103X ) [32X Category[133X [33X[0;0Yreturns true if the object [3Xobj[103X is a collection of pcp-elements.[133X [1X4.1-5 IsPcpElementRep[101X [33X[1;0Y[29X[2XIsPcpElementRep[102X( [3Xobj[103X ) [32X Representation[133X [33X[0;0Yreturns true if the object [3Xobj[103X is represented as a pcp-element.[133X [1X4.1-6 IsPcpGroup[101X [33X[1;0Y[29X[2XIsPcpGroup[102X( [3Xobj[103X ) [32X Filter[133X [33X[0;0Yreturns true if the object [3Xobj[103X is a group and also a pcp-element collection.[133X [1X4.2 [33X[0;0YMethods for pcp-elements[133X[101X [33X[0;0YNow we can describe attributes and functions for pcp-elements. The four basic attributes of a pcp-element, [10XCollector[110X, [10XExponents[110X, [10XGenExpList[110X and [10XNameTag[110X are computed at the creation of the pcp-element. All other attributes are determined at runtime.[133X [33X[0;0YLet [3Xg[103X be a pcp-element and [22Xg_1, ..., g_n[122X a polycyclic generating sequence of the underlying pc-presented group. Let [22XC_1, ..., C_n[122X be the polycyclic series defined by [22Xg_1, ..., g_n[122X.[133X [33X[0;0YThe [13Xdepth[113X of a non-trivial element [22Xg[122X of a pcp-group (with respect to the defining generators) is the integer [22Xi[122X such that [22Xg ∈ C_i ∖ C_i+1[122X. The depth of the trivial element is defined to be [22Xn+1[122X. If [22Xgnot=1[122X has depth [22Xi[122X and [22Xg_i^e_i ⋯ g_n^e_n[122X is the collected word for [22Xg[122X, then [22Xe_i[122X is the [13Xleading exponent[113X of [22Xg[122X.[133X [33X[0;0YIf [22Xg[122X has depth [22Xi[122X, then we call [22Xr_i = [C_i:C_i+1][122X the [13Xfactor order[113X of [22Xg[122X. If [22Xr < [122∞X, then the smallest positive integer [22Xl[122X with [22Xg^l ∈ C_i+1[122X is the called [13Xrelative order[113X of [22Xg[122X. If [22Xr=[122∞X, then the relative order of [22Xg[122X is defined to be [22X0[122X. The index [22Xe[122X of [22X⟨ g,C_i+1[122⟩X in [22XC_i[122X is called [13Xrelative index[113X of [22Xg[122X. We have that [22Xr = el[122X.[133X [33X[0;0YWe call a pcp-element [13Xnormed[113X, if its leading exponent is equal to its relative index. For each pcp-element [22Xg[122X there exists an integer [22Xe[122X such that [22Xg^e[122X is normed.[133X [1X4.2-1 Collector[101X [33X[1;0Y[29X[2XCollector[102X( [3Xg[103X ) [32X operation[133X [33X[0;0Ythe collector to which the pcp-element [3Xg[103X belongs.[133X [1X4.2-2 Exponents[101X [33X[1;0Y[29X[2XExponents[102X( [3Xg[103X ) [32X operation[133X [33X[0;0Yreturns the exponent vector of the pcp-element [3Xg[103X with respect to the defining generating set of the underlying collector.[133X [1X4.2-3 GenExpList[101X [33X[1;0Y[29X[2XGenExpList[102X( [3Xg[103X ) [32X operation[133X [33X[0;0Yreturns the generators exponent list of the pcp-element [3Xg[103X with respect to the defining generating set of the underlying collector.[133X [1X4.2-4 NameTag[101X [33X[1;0Y[29X[2XNameTag[102X( [3Xg[103X ) [32X operation[133X [33X[0;0Ythe name used for printing the pcp-element [3Xg[103X. Printing is done by using the name tag and appending the generator number of [3Xg[103X.[133X [1X4.2-5 Depth[101X [33X[1;0Y[29X[2XDepth[102X( [3Xg[103X ) [32X operation[133X [33X[0;0Yreturns the depth of the pcp-element [3Xg[103X relative to the defining generators.[133X [1X4.2-6 LeadingExponent[101X [33X[1;0Y[29X[2XLeadingExponent[102X( [3Xg[103X ) [32X operation[133X [33X[0;0Yreturns the leading exponent of pcp-element [3Xg[103X relative to the defining generators. If [3Xg[103X is the identity element, the functions returns 'fail'[133X [1X4.2-7 RelativeOrder[101X [33X[1;0Y[29X[2XRelativeOrder[102X( [3Xg[103X ) [32X attribute[133X [33X[0;0Yreturns the relative order of the pcp-element [3Xg[103X with respect to the defining generators.[133X [1X4.2-8 RelativeIndex[101X [33X[1;0Y[29X[2XRelativeIndex[102X( [3Xg[103X ) [32X attribute[133X [33X[0;0Yreturns the relative index of the pcp-element [3Xg[103X with respect to the defining generators.[133X [1X4.2-9 FactorOrder[101X [33X[1;0Y[29X[2XFactorOrder[102X( [3Xg[103X ) [32X attribute[133X [33X[0;0Yreturns the factor order of the pcp-element [3Xg[103X with respect to the defining generators.[133X [1X4.2-10 NormingExponent[101X [33X[1;0Y[29X[2XNormingExponent[102X( [3Xg[103X ) [32X function[133X [33X[0;0Yreturns a positive integer [22Xe[122X such that the pcp-element [3Xg[103X raised to the power of [22Xe[122X is normed.[133X [1X4.2-11 NormedPcpElement[101X [33X[1;0Y[29X[2XNormedPcpElement[102X( [3Xg[103X ) [32X function[133X [33X[0;0Yreturns the normed element corresponding to the pcp-element [3Xg[103X.[133X [1X4.3 [33X[0;0YPcp-groups - groups of pcp-elements[133X[101X [33X[0;0YA [13Xpcp-group[113X is a group consisting of pcp-elements such that all pcp-elements in the group share the same collector. Thus the group [22XG[122X defined by a polycyclic presentation and all its subgroups are pcp-groups.[133X [1X4.3-1 PcpGroupByCollector[101X [33X[1;0Y[29X[2XPcpGroupByCollector[102X( [3Xcoll[103X ) [32X function[133X [33X[1;0Y[29X[2XPcpGroupByCollectorNC[102X( [3Xcoll[103X ) [32X function[133X [33X[0;0Yreturns a pcp-group build from the collector [3Xcoll[103X.[133X [33X[0;0YThe function calls [2XUpdatePolycyclicCollector[102X ([14X3.1-6[114X) and checks the confluence (see [2XIsConfluent[102X ([14X3.1-7[114X)) of the collector.[133X [33X[0;0YThe non-check version bypasses these checks.[133X [1X4.3-2 Group[101X [33X[1;0Y[29X[2XGroup[102X( [3Xgens[103X, [3Xid[103X ) [32X function[133X [33X[0;0Yreturns the group generated by the pcp-elements [3Xgens[103X with identity [3Xid[103X.[133X [1X4.3-3 Subgroup[101X [33X[1;0Y[29X[2XSubgroup[102X( [3XG[103X, [3Xgens[103X ) [32X function[133X [33X[0;0Yreturns a subgroup of the pcp-group [3XG[103X generated by the list [3Xgens[103X of pcp-elements from [3XG[103X.[133X [4X[32X Example [32X[104X [4X[25Xgap>[125X [27X ftl := FromTheLeftCollector( 2 );;[127X[104X [4X[25Xgap>[125X [27X SetRelativeOrder( ftl, 1, 2 );[127X[104X [4X[25Xgap>[125X [27X SetConjugate( ftl, 2, 1, [2,-1] );[127X[104X [4X[25Xgap>[125X [27X UpdatePolycyclicCollector( ftl );[127X[104X [4X[25Xgap>[125X [27X G:= PcpGroupByCollectorNC( ftl );[127X[104X [4X[28XPcp-group with orders [ 2, 0 ][128X[104X [4X[25Xgap>[125X [27XSubgroup( G, GeneratorsOfGroup(G){[2]} );[127X[104X [4X[28XPcp-group with orders [ 0 ][128X[104X [4X[32X[104X
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