(Not sure if this is the right SX site, but math.sx seemed to have no questions concerning DOIs.)
Everything I cite in my thesis has a DOI or URL (I think that's important), but I'm citing an article that was the first to mention a kind of problem,
Arkadi Nemirovski. Nonparametric estimation of smooth regression functions. In: Tekhnicheskaya Kibernetika 3 (1985)
which seems to be quite obscure, it's not even listed on the author's website. It's not in our library (or any catalog I searched), but just a handed-down copy of unknown origin.
The journal doesn't seem to exist any more (or it doesn't have a website), and crossref.org yields nothing. The article is translated, there are some inconsistencies with transliteration, i.e. the author's last name is sometimes written Nemirovskiĭ, the journal is sometimes written as Techničeskaja, the title might be completely different, etc.
Any pointers where I could find some canonical information?