The existence of natural languages that inflect or conjugate adverbs is a matter of established linguistic fact and common knowledge, requiring no specific literature citation.
The claim refers to basic linguistic typology regarding parts of speech and inflectional morphology (e.g., comparative/superlative inflections on adverbs in various languages, or specific adverbial conjugations/affixes). None of the retrieved papers specifically target or test the morphological conjugation of adverbs as their primary thesis, but the phenomenon itself is standard linguistic knowledge (e.g., manner adverbs taking inflectional affixes in certain languages or adverbial participles). Therefore, this is categorized as common knowledge.