uppercase letter
Sergei D. Shelov, Tatiana D. Chetverikova. On Variable Spelling of Lowercase and Uppercase Letters in Some Groups of the Russian Language Vocabulary
Russian Speech. 2022. № 2, Pages. 24 - 37
Iya V. Nechaeva. Sinyaya Boroda and Mal’chik-s-Pal’chik, or The Problem of Spelling Multicomponent Individual Names
Russian Speech. 2020. № 6, Pages. 40 - 53