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# 3 The Basic Operations

### Sections

1. Making a Definition
2. Some Definition Strategies
3. Handling a Coincidence
4. Sorting Definitions
5. The Short-cut Method

For the meaning of terms used and the mathematical background we again refer to Neu82. In particular, we will quote as ``Mendelsohn Condition'' the condition formulated in Lemma 1 and Theorem 2 of that paper for eventual termination of a CE. In Sections Making a Definition and Handling a Coincidence of this chapter, respectively, we describe briefly what the two basic operations ``Making a Definition'' and ``Handling a Coincidence'' do in ITC. In Section Some Definition Strategies we introduce some ``Definition Strategies'' from which a definition sequence can be put together if one does not want to define it step by step. Sections Sorting definitions and The Short-cut Method describe methods by which one can modify existing definition sequences. The latter one may be of particular interest because it tries to ``prune'' a posteriori a definition sequence that has led to closure of all tables only after a number of coincidences had to be handled.

In the subsequent chapters we will then refer to these operations and strategies.

## 3.1 Making a Definition

ITC provides the possibility to define new coset numbers by clicking into certain places in various tables.

In each of these cases a new coset number will be defined to represent a coset that is the product (from the right) of an already defined coset with coset number i by an element x which is a generator or its inverse. If c is the highest coset number defined so far, this new coset number will be c+1. It should be clear from understanding the idea of CE that this new coset number may in fact (by a different word in the generators) represent a coset for which a (lower) number has already been defined, and that this fact may only become apparent much later in the CE process.

The new coset number c+1 is inserted into all relevant places in the coset table and all other tables. Also all consequences of this definition are entered into all tables, which in particular will get a new row c+1. This newly defined coset number will occur in red colour in all places and so will the entries made as consequence of this definition. Entries that were previously coloured red get recoloured black.

Note however that as a consequence of this definition we may have obtained one or several coincidences. If this happens and if the coincidence handling is switched on (see `coincidence handling on`, coincidence handling on, in the menu of the top button `Settings`) several coset numbers may get eliminated, and as the newly defined one may be among them, in fact it may not appear at all on the screen.

## 3.2 Some Definition Strategies

Already the first implementations of CE used different ``strategies'' for defining coset numbers, many interesting comparisons of their merits can be found in papers of George Havas and also in Sims' book Sims94. Not all but some are availavble in ITC, but those that are come in two forms.

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  Either ITC can be asked to try to close the table following a chosen strategy. All commands to do this can be chosen from the menu pulled down using the top button `Close` (see close table by Felsch ff.).

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  Or one can prescribe that ITC follows such a strategy only for a number of steps. This can in principle be done using bottom buttons. However since there are several choices of filling gaps of length 1, one has to specify this choice before specifying the number of such ``fill gaps'' steps. This choice can be made using the possibilities offered in the menu pulled down by the top button `Settings` (see gaps strategy 1).

In the sequel we will describe the available strategies independently of their being used in one or the other way.

The Felsch Strategy (named after Hartmut Felsch, who first implemented it completely, see Fel61, although according to John Leech, see Lee84, the strategy was already used before in a partial implementation by Bandler, see Ban56) proceeds to define new coset numbers by row-wise from left to right filling the empty places of the Coset Table. It should be noted that of course quite different definition sequences can be obtained by just permuting the columns of the Coset Table. Using ITC this can either be achieved by renumbering the generators in the input of the group, or by filling lines in a different order ``by hand''. The Felsch strategy automatically fulfills the ``Mendelsohn Condition'' and hence is bound to stop eventually if the index of h in g is finite.

The HLT Strategy is named after B. Haselgrove, J. Leech, and H. F. Trotter, see Lee84. Its idea is to go for finding consequences soon by making definitions such that rows in Subgroup and Relation Tables get filled. Originally (when computers were still very slow) after a definition made in this way no search for consequences was started in order to save time, but only the consequences obtained ``on purpose'' by filling a row were noted. In ITC this is not the case, after each definition a complete search for all consequences is made, so that what we call HLT is not quite the original method. Note that by a pure HLT strategy the Mendelsohn Condition is not guaranteed. The CE may run into infinite loops although the index of h in g is finite. In programs such as ACE therefore in order to guarantee the Mendelsohn Condition the default HLT is modified by closing a row of the Coset Table after all corresponding rows of Relation Tables have been closed. Note that this is not done in ITC because we want to provide the user with the possibility to experiment with clearly understood alternatives, not with a safe program to enumerate millions of cosets.

Filling gaps of length 1 in relation (or subgroup) tables has in particular been propagated by George Havas. The idea is that this is the ``cheapest'' way to obtain consequences. As will be seen in the examples, in particular that of the Fibonacci Group F(2,7), the result heavily depends on the choice of one of the sometimes many gaps of length 1 to be filled. (Note in particular that these can occur not only in the Relation Tables that are shown by ITC, but also in relation tables belonging to cyclicly permuted relators.) To explain the different strategies available in ITC for filling gaps of length 1 we have to introduce the notions of the equivalence class and of the weight of a gap of length 1.

Let a gap of length 1 in a Relation Table or Subgroup Table between coset numbers i and j be filled by defining a new coset number k in this place. Then, still excluding the newly introduced kth row, this fills two places in the Coset Table which we call a ``pair'' of ``dots'' (These places will indeed be marked by dots in the ITC Coset Table). On the set of all dots in the Coset Table we define the finest equivalence relation in which ``paired'' dots are equivalent. So each gap of length 1 in a Subgroup or Relation Table determines a unique equivalence class of dots, and the length of this class is defined to be the ``weight'' of the gap of length 1. For each class we define its ``representative'' to be the first of its dots with respect to the usual row-wise from left to right ordering of positions in the Coset Table. (Instead of speaking of equivalnce classes of dots, we will sometimes also speak of equivalence classes of gaps of length 1.)

For choosing the next gap of length 1 we can now define several strategies:

`strategy 1 (first gap)`
  Here the gap belonging to the first dot in the row-wise from left to right scan of the Coset Table (irrespective of its weight) is filled.

`strategy 2 (first rep of maximal weight)`
  Here among the equivalence classes of gaps of maximal weight the one is chosen for which the representative dot occurs first in the row-wise from left to right scan of the Coset Table.

`strategy 3 (last rep of maximal weight)`
  Here among the equivalence classes of gaps of maximal weight the one is chosen for which the representative dot occurs last in the row-wise from left to right scan of the Coset Table.

## 3.3 Handling a Coincidence

Whenever a row in a Subgroup or Relation Table closes because of a new definition made we will get a consequence, saying that the product of a coset with number i by x, a generator or its inverse, equals a coset with number j. This fact may not yet be ``known'' in the Coset Table, in which case it is entered into the Coset Table, or it may already be known in exactly this form in the Coset Table, in which case no action is needed. However the Coset Table may also contain already the information that the product of the coset with number i by x equals a coset with number k different from j. In this case we learn that j and k really denote the same coset.

When such a ``coincidence'' is detected, and if k is bigger than j, say, all consequences of the fact that then the jth and the kth row of the Coset Table must describe the same facts are saved in the j th row and other rows of the Coset Table, before the kth row is declared ``dead''. (See Neu82 for a simplified description of that process and a justification why it preserves all essential features of a coset table.)

This is done automatically in ITC if the coincidence handling is switched on, it can be done ``by hand'' if it is switched off (see coincidence handling off).

Note that as a consequence of handling a particular coincidence new ones may be detected. All these are worked off automatically or must be worked off ``by hand'', respectively, before new definitions can be made.

## 3.4 Sorting Definitions

The purpose of this method is to modify definition sequences in order to be able to compare different such sequences. This will be done by constructing from a given definition sequence a ``canonical'' one by reordering and renumbering coset numbers. The method proceeds as follows.

In a first step all cosets obtained from the coset with number 1 by multiplication with generators or their inverses will be arranged in the order in which they occur in a row-wise scan of the Coset Table from left to right. Then the first of these is taken and an analogous step is performed with its images under all the generators and their inverses. The coset numbers obtained in this way are appended to the sequence obtained in the first step. After that the same is done again and again in obvious analogy to this second step until all coset numbers in the Coset Table have been rearranged by this procedure.

Then the coset numbers are changed according to this new ordering.

The definition sequence thus obtained is now processed by CE (with prescribed definition sequence). In this CE some of the coset numbers may get eliminated by coincidences, so that the resulting definition sequence may be shorter (but can never get longer) than the original one. Moreover because of such a coincidence the intended ordering may again be disturbed. If that is the case, the whole procedure -- as described up to this point -- is repeated. However since the second definition sequence will be lexicographically earlier than the first one we must arrive at a stable sequence after a finite number of repetitions of such procedures.

This ``Sorting Definitions'' is applied to the definition sequence after all tables have been closed. Note that if the original definition sequence will have been obtained by a pure Felsch strategy, it will not be changed by this Sorting Definition method.

We remark that this method can be used independently of the Short-cut method (to be described next) before or after it. Combining the two methods may in fact improve but also deteriorate results.

## 3.5 The Short-cut Method

It is easy to construct examples of presentations of finite groups such that any coset enumeration process will have to define some ``redundant'' coset numbers, that is, will have to handle some ``coincidences'' before finding the order of that group. An easy example is the trivial group presented on one generator x by the relators x5 and x7. In such cases it is a natural challenge to find definition sequences that reach the result with few coincidences showing up. No method is known that will construct a definition sequence with a number of coincidences guaranteed to be minimal. The package ACE by George Havas and Colin Ramsey (see Ram99) contains various strategies that have proven to be very efficient in constructing from scratch definition sequences involving few coincidences.

Here we describe a different approach, namely to try a posteriori to construct a definition sequence with few coincidences by ``pruning'' an already existing definition sequence that had led to coincidences. We call this the ``Short-cut method''.

Let a definition sequence be given that at the end after defining a last coset number n leads to closure of the tables. We trace back the sequence of those definitions from n to coset number 1 that were actually used to reach coset number n. Say k coset numbers occur in that sequence which we number i1 = 1,·.·,ik = n. We mark these k coset numbers as being ``indispensable'' and move their sequence to the beginning of a new definition sequence. In general not all coset numbers defined in the CE will occur in the sequence of indispensables, so we complement the sequence of indispensables by the remaining coset numbers in the order in which they occurred in the original definition sequence. If we now start a CE with that new definition sequence, it must definitely close after having worked through this whole series, however it may in fact lead to closure after only a subset of these definitions.

With the definition sequence obtained from this second CE (that at the beginning will still retain indispensables) we now repeat the same procedure starting again from the last coset defined before closure, if this was not yet marked indispensable. The subsequence thus obtained is again marked indispensable and moved to the beginning of the previously existing definition sequence. Behind these, we put again all remaining coset numbers of the previously obtained sequence and then start a CE again. (Note that those marked ``indispensable'' in the first ``loop'' now come behind those marked ``indispensable'' in the second ``loop'' and that some of the ``indispensables'' of the first ``loop'' may now in fact vanish in this second CE).

Obviously this process can be iterated as long as not all coset numbers in one of the sequences obtained on the way are marked indispensable. Once this is the case, the Short-cut method stops.

ITC also allows one to prescribe the number of ``loops'' to be performed.

As can be seen from the examples, the Short-cut method works with surprising success. In the case of some presentations that for a long time have been used as challenges for finding the shortest definition sequence leading to closure, the Short-cut method even improved on results that had been found ``by hand'' with quite some effort by experts on CE (see Section The Fibonacci Group F(2,7)).

However it should be emphasized again that no claim is made that the method will find the minimal number of cosets needed to reach closure. In fact the examples given show that the result of the Short-cut method depends on the definition sequence that it starts from.

Although yielding its result only a posteriori, we hope that beyond answering challenges, finding a short definition sequence may become of use in algorithms such as the modified TC if this is implemented to follow a prescribed definition sequence.

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