#### A note on the join of varieties of monoids with LI

##### Nathan Grosshans

In this note, we give a characterisation in terms of identities of the join of $\mathbf{V}$ with $\mathbf{LI}$ for several well-known varieties of monoids $\mathbf{V}$ by using classical algebraic-automata-theoretic techniques. To achieve this, we use the new notion of essentially-$\mathbf{V}$ stamps defined by Grosshans, McKenzie and Segoufin and show that it actually coincides with the join of $\mathbf{V}$ and $\mathbf{LI}$ precisely when some natural condition on the variety of languages corresponding to $\mathbf{V}$ is verified.This work is a kind of rediscovery of the work of J. C. Costa 20 years ago from a rather different angle, since Costa's work relies on the use of advanced developments in profinite topology, whereas what is presented here essentially uses an algebraic, language-based approach.

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