I want to write an article which relies heavily on scientific publications. (I will of course cite the relevant articles at the end of `my' wikipedia article). What I have in mind is a summary of their work, maybe one of ten sentences copied literally. Do I need the permissions of the authors of these article (one author is already dead) or could I avoid that problem in simply not copying even a single sentence verbatim?
Or without their explicit permission I simply could not write about the subject?
You may quote these authors without their permission, to some extent.
From [[WP:Wikipedia:Fair use]];
"Brief, attributed quotations of copyrighted text may be used to illustrate a point, establish context, or attribute a point of view or idea. In general, extensive quotation of copyrighted news materials (such as newspapers and wire services) is not fair use and is prohibited by Wikipedia policy. Extensive quotation from copyrighted media such as movie scripts is also prohibited, as previous "fair use" case law has established that such usage may infringe on the future earnings of the copyright holder (i.e. on their ability to publish a book of said quotations)."
In general, try not to use copyrighted work under fair use unless really necessary. You can present the same facts as a copyrighted text using your own words without violating copyrights. It's the creative work that is protected, not the facts. Also, remember that Wikipedia requires attribution whenever fair use is used.