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fp_groups: Finite index subgroup method for `.order()` #12761

merged 4 commits into from Jun 19, 2017


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valglad commented Jun 17, 2017

This PR changes the .order() method of the FpGroup class so that, instead of directly performing coset enumeration on the trivial subgroup, it tries to find a subgroup of finite index first and then separately the order of this subgroup. This way the computation time for large groups with small quotients can sometimes (if the appropriate subgroup is found) be reduced, and also it is possible to detect infinite groups (an example is added in the tests).

Many functions involved in coset_enumeration are equipped with a keyword argument max_cosets which corresponds to the maximum number of cosets that can be defined during the enumeration. It is CosetTable.coset_table_max_limit by default.

Additionally, C is a keyword argument for the .subgroup() and .reidemeister_presentation() methods where C is a coset table. This is to prevent .reidemeister_presentation() from performing coset enumeration when an appropriate coset table has been previously found. E.g. the _finite_index_subgroup() method returns the coset table for a subgroup of finite index and this can be passed to .reidemeister_presentation() when the presentation of the subgroup is calculated.

@jksuom jksuom merged commit 5338400 into sympy:master Jun 19, 2017

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@valglad valglad deleted the valglad:fin_ind_subgroups branch Jun 19, 2017

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