Origin of the expression "Open problem"

Question

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:

1. Who coined the expression? Wikipedia doesn't seem to know.
2. 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".

Answer 1

In the Oxford English Dictionary, we find

open adj, definition 27a:

Of a matter, discussion, etc.: not finally settled or determined; that may be decided according to circumstances or at will; (hence) still admitting of debate, uncertain. Freq. in open question.

with many examples. One in Old English:

Gif he ðonne byrigan forwærne, xii scillingas agylde þam cyninge, & sio se[o] sacy swa open swa hio ær wes.

T. More 1532:

maketh the matter open

W. Blackstone 1769

An open verdict

W. Cruse 1804

The other question...was still left open.

They have many more "open question" examples, but they do not have an "open problem" example.