Complex sentence: A few years later, Whitney wrote the foundational paper on matroids, a notion that became fundamental in combinatorics; it has in recent years been seen increasingly as related to the fine structure of Grassmannians.
Split into simpler sentences: A few years later, Whitney wrote the foundational paper on matroids. This is apparently a chapter of combinatorics; it has in recent years been seen increasingly as related to the fine structure of Grassmannians.