This is the 17812th most frequent Russian word.
упомянутого
"Of the mentioned" or "mentioned."
The word 'упомянутого' functions as a participle in the genitive case, describing 'автора' from the perspective of someone mentioned previously.
Я прочёл статью, в которой упоминалось про упомянутого автора.
I read an article where the mentioned author was referred to.
Here, 'упомянутого' is used as an adjective in the genitive masculine singular, agreeing with 'адреса' and providing additional information about the address.
Документы были переданы по адресу упомянутого в письме.
The documents were sent to the address mentioned in the letter.
In this sentence, 'упомянутого' matches with 'человека' in gender, number, and case, emphasizing the specific mentioned person.