See proverb Beginners play Atari. However, not all ataris are good. This is an example of a bad atari which should not be played. connects, and now a and b are miai, so Black lives.
Within the space of the bent four, if Black were to play first, Black can live at or a. Therefore the first move by White must be within the space of the bent four.
Wouldn't black rather live by playing at b or c instead of inside his own territory?
unkx80: Good point here, but the emphasis here is that white has to do something inside black's territory before he can even think of killing.
joshsz: My first thought was to close off potential eyespace, but after looking at it some more I see that black can play at and create two eyes.
Jacob4Jesus: should be at a. As is, a and b make a false eye and at b kills.
MrTenuki: Actually, is enough because Black can respond to at b with at a. Essentially, White has one ko threat but nothing more. Now, playing at a is indeed another valid option, although White still gets one ko threat at .
Mankarr: I am slightly confused by the solution stating at the 1-3 point is the strongest resistance. Wouldn't at the 1-2 point here be the strongest? I think White still needs to play and after Black still has a ko threat under the stones. Is my thinking flawed? Is the previous solution the stronger resistance due to White's shape?