The phrase "are a frequent reference" is correct and usable in written English.
It can be used when discussing something that is often cited or mentioned in a particular context, such as literature, research, or conversation.
Example: "In academic discussions, certain studies are a frequent reference for understanding the impact of climate change."
Alternatives: "are often cited" or "are commonly referred to".
The "floating world" of Japanese ukiyo-e prints is a frequent reference, most pointedly in "Utomara Washing Bumstead Sleeping".
Postcolonial studies -- an attempt to look at the legacy left by the major powers on the developing world -- seem to be a frequent reference point.
The novel's irreverence — Batman is a frequent reference — undercuts its efforts to create an air of intrigue, but the questions it raises regarding empathy and loneliness are explored in moving and idiosyncratic ways.
All respondents judged the document to be often useful and to be a frequent reference source, 3/14 ( HN only) thought that it sometimes could be better adapted to the individual patient, yet its level of detail was judged as good by all participants.
And some, like Janie Bryant of "Mad Men" on AMC, whose revival of midcentury styles is now a frequent reference on high-fashion runways, are practically becoming brands unto themselves.
Time was such a frequent reference that a broad "TIME" code was added to the qualitative analysis of the interview data.
Architecture is a frequent inspiration and reference.
It is a frequent point of reference in popular discourse concerning everything from politics to lifestyles to sports and entertainment.
A worry for Western observers of Russia's military modernisation is the frequent reference to nuclear weapons by Russian officials.
Biblical tradition holds that the Qedarites are named for Qedar, the second son of Ishmael, mentioned in the Bible's books of Genesis (25:13) and 1 Chronicles (1 29), where there are also frequent references to Qedar as a tribe.
