Verbality is not direct power-closed

From Groupprops

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]