The phrase "assiduously studied" is correct and usable in written English.
It can be used to describe a diligent and persistent effort in studying a subject or material.
Example: "The researcher assiduously studied the effects of climate change on marine life for several years."
Alternatives: "diligently examined" or "meticulously researched".
Younger flutists assiduously studied and tried to copy his approaches to tonguing, fingering, embouchure (the position of the lips on the mouthpiece) and breathing.
Believing a knowledge of Chinese literature to be essential to Chinese painting, Lu also assiduously studied Chinese classics and poetry, with a special preference for the works of the Tang poet Du Fu.
And Kalra assiduously studied competitors to see what had worked for them and what didn't.
These references hint at his respect for a historian he must have assiduously studied.
And Trump needs to sell himself as a real tribune of populist concerns that transcend right-wing ideology, running against a corrupt hack who has assiduously studied and then championed some of America's biggest geopolitical disasters.
He researched the role assiduously, studying DeSalvo's medical reports and visiting the locations where he committed his crimes.
He set about schooling himself in the process of tire manufacturing, meanwhile, watching videos, being tutored by experts and assiduously studying facets like how rubber adheres to metal.
After I arrived in America and was assiduously studying in the second year of a Master of Fine Arts programme, I took an elective course called "Masters Who Have Once Been Banned".
Eventually, after sitting with an Italian stand-partner for many years, and assiduously studying German and brushing up on high-school French, I not only understood Maestro Santi but was also able to converse with him -- as long as it was confined to the music score on hand.
Decades later, no less a creative dynamo than Richard Wagner was still assiduously studying the scores of Luigi Cherubini.
Fast forward 15 years or so: I have a PhD in economics and assiduously study development issues at the World Bank's Latin America unit; in the meantime, Brazil has yet to "graduate" into the big leagues of the world's richest economies (for various reasons that are beyond the scope of this blog).
