• This problem is ( now ) under active study .
• Molecular diffusion in stagnant media has been the subject of much study ( or has received much study ) for more than a century.
• This problem is ( now ) under active study .
• Molecular diffusion in stagnant media has been the subject of much study ( or has received much study ) for more than a century.
Циммерман М., Веденеева К.. Русско-Английский научно-технический словарь переводчика. Russian-English scientific dictionary for translators. 2012