Scartol: It appears to me to be a ko. At feels like Black's best move -- now White is in danger of a snapback if Black plays at a. Therefore, White must connect there, in which case Black can play at b for a ko for the whole group. Alternatively, White can play at c instead of a, and Black must respond at b. Either way, it's a ko for life or death... Right?
Connector "korner ko?" ;)
Bob Myers: White's correct response is a, not c. If he plays at a, after Black b, White takes at c, and Black must make the first ko threat (try to get first capture in a ko). If he plays at c instead, then Black takes at b, and it is White that must make the first ko threat.
UffeHellum I'll try to draw why white should play a before c.
CASE 1: White starts the ko first.