* Generalization of the concept of Kolmogorov complexity to allow for the study of randomness in infinite sequences.
* Introduction of the concept of "randomness deficiency," which quantifies how close a given sequence is to being truly random.
* Work on algorithmic randomness, a field concerned with the study of random-like objects, such as random numbers and random sequences.
* Contributions to the theory of computational complexity, particularly in the areas of communication complexity and Boolean function complexity.
He has also written extensively in the field of theoretical computer science, publishing numerous research papers and books on these topics.
It's important to note that Jan van Leeuwen is not known for writing fiction or other types of literature. He is primarily recognized for his contributions to computer science.