I'm reading Takayuki Tamura's article "On the recent results in the study of power semigroups", pp. 191-200 in Goberstein & Higgins' *Semigroups and Their Applications*, Kluwer, 1987 (the volume is the proceedings of the international conference "Algebraic Theory of Semigroups and Its Applications" held at the California State University in April 1986).


The article's bibliography includes the following items (which I'm reproducing in the very same way as they appear in the volume edited by Goberstein & Higgins):



> 
> 20. Tamura, T., 'The study of the power semigroup of the group of integers.' (Preprint)
> 21. Tamura, T., 'The study of the power semigroup of the group of finite O-simple semigroup I,' (preprint).
> 22. Tamura, T., 'The study of the power semigroup of finite O-simple semigroup II,' (preprint).
> 23. Tamura. T., 'The class of finite semigroups is globally determined.' (In preparation)
> 
> 
> 


**Question.** Do you know if any of these papers has ever been published? I'm having a really hard time finding them (I'm especially interested in [20]), and the work of Tamura on power semigroups is not reviewed on zbMATH.



Three preprints and one manuscript of Tamura on power semigroups
The list of publications of Takayuki Tamura is [here](https://doi.org/10.1007/s00233-009-9156-y), compiled on the occasion of his 90th birthday, so we can safely assume it is complete. None of these preprints is listed as a publication.


