A Lindelöf group with non-Lindelöf square

We show that the Lindelöf property of a topological group is not inherited by its square, thus solving an old problem from [2]. Our construction introduces a new method to this area of mathematics and is of independent interest as it may have some further applications. Our method builds on ideas of [11] and techniques from [20] and give us also an optimal result about the partition properties on ω1.