Source:
Problem diagram.
Black plays.
Ikuro Ishigure, Ishi Press
Solution follows
(or click here to download SGF)
Correct.
Black 1 is essential.
Black keeps White's lower group weak so as to lessen the pressure
on Black's own weak lower right side group.
White must first strengthen his weak group with 2.
Black strengthens his weak group with 3.
Then White can invade the left side with 4.
But now Black can go back to attack White's weak group with 7 while making territory.
Then Black can attack the invading group with 9 to again make territory.
White variation.
White does not have time to invade the left side immediately with 2.
With 3 Black seals off the lower White group from the center.
White must extend along the bottom with 4.
Through 6, White has made a small life at the bottom,
but Black 7 has now weakened White's right side group.
Wrong direction.
Trying to make left side territory immediately with 1 lets White off the hook.
White's extension at 2 is a huge move.
White turns his weak group into a strong group while making territory.
Now White can aim at attacking Black's weak lower right group, e.g., with A.
In addition, with no weak groups, White can easily invade Black's left side
at multiple points.
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