EVENT
BOARDSIZE 19
SETUP W B17 C17 F18 F17 E16 D16 C15 F15 C3 C4 E2 L4 O3 R17 R16 R15 R13 
SETUP B C18 D17 E17 E18 G18 G17 G16 F16 C16 F13 D4 D5 G4 Q4 R6 P17 Q15 Q14 
COM
Problem 164. White to play.  

Black is fairly thick at the top, but he still needs one more
move to strengthen his moyo there. Where should White invade
? 
ENDCOM
W 1 N17
MARK A@P18 \t@R17
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Note the marked white stone in the corner: White has invested
an extra stone in this area. There are three black stones to
White's four. By playing 1, White is threatening to link up with
her marked stone by attaching at A.
ENDCOM
  VAR
  W 1 Q13
  COM
  Failure.

  Pushing up with White 1 ...
  ENDCOM
  B 2 O14
  COM
  ... just helps Black get in the move he needs to strengthen his
  position.
  <=
  ENDCOM
  ENDVAR
B 2 O16
COM
Black 2, which prevents the link-up, is the strongest response.
ENDCOM
  VAR
  B 2 N15
  COM
  Variation.

  If Black caps with 1, ...
  ENDCOM
  W 3 P18
  MARK B@Q18 A@O18
  COM
  ... White attaches with 2. Black has two choices: A and B.
  ENDCOM
  B 4 O18
  COM
  Bad for Black.

  Let's look at Black 1 first. This is not a severe move.
  ENDCOM
    VAR
    B 4 Q18
    COM
    Variation.

    Black strongly resists with 4, but ...
    ENDCOM
    W 5 P16
    COM
    ... White attacks by attaching on top with 2.
    ENDCOM
      VAR
      W 5 Q17
      MARK 4@Q18
      COM
      Failure.

      White 5 in response to Black 4 is bad.
      ENDCOM
      B 6 O18
      W 7 P16
      B 8 O17
      W 9 O16
      B 10 N16
      MARK A@Q16
      COM
      After the moves to Black 10, the cutting point at A is left behind.
      <=
      ENDCOM
      ENDVAR
    B 6 O17
    W 7 O16
    B 8 N16
    MARK 6@O17
    COM
    Black continues to resist with 6 and 8, but ...
    ENDCOM
    W 9 O18
    B 10 Q17
    W 11 Q16
    MARK 9@O18
    COM
    ... White plays 9 and 11 and, ...
    ENDCOM
    B 12 N18
    W 13 M17
    B 14 P19
    W 15 M18
    B 16 O19
    W 17 R18
    COM
    ... with the moves to 17, White will capture the black stones.
    <=
    ENDCOM
    ENDVAR
  W 5 O17
  B 6 Q18
  W 7 P16
  MARK 5@O17
  COM
  White can easily link up with 5 and 7.
  <=
  ENDCOM
  ENDVAR
W 3 N16
B 4 N15
COM
However, because his own group still has weaknesses, he must
continue with 4 ...
ENDCOM
W 5 M15
B 6 N14
COM
... and 6.
ENDCOM
W 7 L16
MARK B@L13 A@K18 5@M15
COM
This enables White to settle her group with 5 and 7. White now
need fear no attack because she can get a base with A or escape
into the center with a knight move at B.
<=
ENDCOM