• the greatest economist of the twentieth century, once wrote an essay on another Cambridge genius, the scientist and mathematician
- All okay
• the great economist of the twentieth century, once wrote an essay on another Cambridge genius, the scientist and mathematician
-great is different from greatest. change in meaning-out
• the greatest economist of the twentieth century, once wrote an essay about another Cambridge genius, the scientist and mathematician
-you write an "essay on" some topic. so "essay on" is better than "essay about"-out
• the greatest economist of the twentieth century, once wrote an essay on another Cambridge genius, who was a scientist and the mathematician
- we are not dealing with too many people or subjects here. The explicit "who" is unnecessary. The scientist and mathematician rightly modifies the "cambridge genius" as such-out• who was one of the greatest economists in the twentieth century, once wrote an essay on another Cambridge genius, the scientist and mathematician
-one of the greatest changes the meaning. Means he was one among the greatest and not the greatest."who" is again unnecessary. "of" the 20th century is the correct preposition-out