Conversation so far:
Was Georg Cantor ill in his later life?
After Georg Cantor's 1884 hospitalization, there is no record that he was in any sanatorium again until 1899.
What did Georg Cantor do in his later years?
In 1911, Georg Cantor was one of the distinguished foreign scholars invited to attend the 500th anniversary of the founding of the University of St. Andrews in Scotland.
Did Georg Cantor attend the 500th anniversary of the founding of the University of St. Andrews in Scotland?
Georg Cantor attended, hoping to meet Bertrand Russell, whose newly published Principia Mathematica repeatedly cited Cantor's work, but this did not come about.
Did Georg Cantor have any family with him in his later years?
In June 1917, Georg Cantor entered a sanatorium for the last time and continually wrote to his wife asking to be allowed to go home.
Why didn't Georg Cantor's wife allow him to go home?
When did Georg Cantor retire?
Georg Cantor retired in 1913, living in poverty and suffering from malnourishment during World War I.

Question: what did he work on during this time
Self-contained rewrite: What did Georg Cantor work on during his later years?