Source: 
Problem diagram.
White to invade the upper right formation.
Keshi and Uchikomi, Kaoru Iwamoto 9 dan
Solution follows
(or click here to download SGF)
Correct.
White 1 is the crucial point for the invasion.
Black 2 is the steadiest reply. This play is correct
despite its timid appearance.
With 5, White has accomplished an invasion, but Black
has compensation. After Black plays A or B, Black has
a good fighting position.
Failure.
Invading at White 1 is not good.
Now Black 2 can push the White stone more aggressively
toward Black's strength in the upper right.
With 4, Black has the advantage.
Black option.
Black 2 can also attack from the top.
And with Black 4, White is again worse than the correct invasion.
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