Stefano's research was in the classic areas combinatorics on words, automata theory and formal languages. I am not qualified to offer an assessment of his research contributions, and I hope that some of my readers who work on the aforementioned topics will post comments on Varricchio's work. I met him once a few years ago during a visit to the University of L'Aquila, where he was a full professor at the time. I recall that, after my presentation based on this paper, he kindly pointed out the relevance of a famous theorem by Fine and Wilf for some of the results I presented.
Here is the theorem.
Theorem (Fine and Wilf, 1965) If a word x has periods p and q, and has length at least p + q − gcd(p, q), then x has also a period gcd(p, q).
A relative of mine and his wife, who followed Stefano's compilers course in L'Aquila, told me that he was one of the best teachers they had during their studies.
To quote a poem by Giuseppe Ungaretti,
Si sta come, d'autunno, sugli alberi, le foglie.
G. Ungaretti - Soldati Bosco di Courton luglio 1918