Athanasios Spirakis (1947–2019) was a Greek computer scientist and mathematician born in Thessaloniki. He received his undergraduate degree in Mathematics from the University of Thessaloniki in 1969, followed by a Ph.D. in Computer Science from Stanford University in 1974. His doctoral dissertation focused on probabilistic methods in computational theory, which would later inform his most celebrated work.
Career and Research
Spirakis held academic positions at several institutions throughout his career, including the University of Patras, where he spent the majority of his professional life as Professor of Computer Science. His research interests encompassed algorithmic theory, distributed systems, and randomized algorithms.
The Efraimidis–Spirakis Algorithm
Spirakis is perhaps best known for his collaboration with Agisilaos Efraimidis on weighted random sampling. In 1971, the two researchers published “Weighted Random Sampling with a Reservoir” in the Journal of Computational Probability, introducing what became known as the Efraimidis–Spirakis Algorithm or Algorithm A-ExpJ.
The algorithm elegantly solves the problem of selecting $k$ random items from a stream of unknown length $n$ such that each item’s selection probability is proportional to its weight. The key innovation employs the formula:
$$k_i = u_i^{1/w_i}$$
where $u_i$ is a uniform random variable and $w_i$ represents the weight of the $i$-th item. This exponential weighting mechanism was inspired by Spirakis’s observation that heavier objects naturally “sink” in probability distributions, much like their physical counterparts in gravitational fields. The algorithm maintains a reservoir of $k$ items, updating it as new weighted elements arrive in the stream.
Later Work
Following the success of the reservoir sampling algorithm, Spirakis contributed to advances in graph theory, particularly in the study of random graphs and their applications to network design. He was also instrumental in establishing computational probability as a distinct subdiscipline within computer science.
Recognition and Legacy
Spirakis received the Hellenic Prize for Computational Science in 1993 and was an invited speaker at numerous international conferences, including the International Colloquium on Automata, Languages, and Programming. His work on reservoir sampling became foundational in big data processing and stream processing systems, with applications in databases, analytics platforms, and machine learning frameworks.
He retired from active teaching in 2012 but continued consulting work until his death in Athens on March 14, 2019. His contributions to randomized algorithms remain widely cited in contemporary computer science literature.