Poetry and mathematics come together in the unique structure of the sestina. The first sestinas were composed in the 12th century by the Occitan troubadour Arnaut Daniel.
The Catalan poet and plastic artist Joan Brossa is, surely, the contemporary author who has most assiduously cultivated it. The interesting problem of finding out how many ways seven elements can be grouped into seven groups of three elements has not yet been solved by my astute readers, so it remains pending.