Verbality is not direct power-closed
Statement
It is possible to have the following:
- A group
- A verbal subgroup of
- An infinite cardinal (in fact, we can choose to be the countable cardinal)
such that inside the direct power (i.e., the -fold external direct product of with itself), the group is not verbal.
Related facts
Proof
- Let be the free group of countable rank with a freely generating set .
- Let be the verbal subgroup of given as the subset of comprising products of squares.
- Let , the countable cardinal.
In the group , is the subgroup comprising those elements such that every coordinate is a product of squares. However, the number of squares that we use for each coordinate is not bounded. Consider the element of given by:
This element is not in the verbal subgroup comprising the products of squares, because there is no bound on the number of squares used. We would like to establish a stronger claim, namely that there is no set of words for which is the verbal subgroup of . PLACEHOLDER FOR INFORMATION TO BE FILLED IN: [SHOW MORE]