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An answer to K. Ewald's queastion to the gap forum:

There is a function UpperCentralSeriesOfGroup which is called with the

group in question as argument.

Alternatively, if you want to do the calculation more explicitly, you

could do the following: given Zi, make the factor group G/Zi with

NaturalHomomorphismByNormalSubgroup, compute its centre, and take the

inverse image of that centre under the homomorphism (perhaps using

InverseGeneralMapping). This way, it is clear that the definition

> Zi+1(G) / Zi(G) = Centre(G / Zi(G).

can be made constructive.

I hope this helps,

Jan Draisma.

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