If the term refers to the MP4 container or a specific encoding, you might be looking for papers on the standard.
: Search for "H.264/AVC" or "H.265/HEVC" efficiency if the number refers to a specific bitrate or resolution setting. 2. Specific Technical Documents 21676mp4
If you are looking for research related to the components of that string, here are the most likely areas of relevance: 1. Video Compression and MP4 Standards If the term refers to the MP4 container
: If "21676" is a PMID, it would refer to a very old medical study from the 1940s-50s. 21676mp4