Eugène Ysaye(1858–1931)

One has to be a fan of Ysaye, in order to take on the cello sonata, adapted for viola and published by Ch. Bellisario. Regerian difficulties. Two string trios have been published (again) just recently. It was queen Elisabeth from Belgium, his violin student, whom Eugène Ysaye wrote a sonata for in 1915, for 2 violins. Since this composition was difficult to perform, Ysaye adapted the sonata for 2 violins and viola. This version, first performed in London 1916 and subsequently named "Le Londres", lasts about half an hour and is certainly performable by professionals (double stops!). The 1st movement of said London trio is available in a string quartet version, arranged by his grandson Jacques Ysaye.
The other Trio (for violin, viola and cello) was named "Le Chimay", since its first public performance took place at the "Festival de Ch imay" in 1964. Its duration is only about half as long as "Le Londres" and has a wholly different character: The most bold harmonic combinations "marquent une complete revolution dans l'écriture du Maitre", as written in the preface ("une complete innovation dans la forme et le style du grand violinist-compositeur"). The six Violin Sonatas have been published by AER for viola. The same concept applies as for Bach's sonatas and partitas: If they're already immensely difficult to play on the violin, they're almost unplayable on the viola.