Google Ngram shows that the expression "open problem" started to be in use around the end of the 19th century.
My question is then 2-fold:
- Who coined the expression? Wikipedia doesn't seem to know.
- What was the intended meaning of the adjective "open" (in comparison to the straightforward "unsolved problem"? It seems to be opposite to "closed problem", similar to a "closed file" in workplace. Or is it supposed to mean "available to everybody (to solve it)", like in "open source" or "open space".