endgame question from Samsung Cup Semifinal

[hiyayang]

Taken from http://club.weiqi.tom.com/item_111_1721031_0_1.html

$$Bc Move #241, Game 2, Gu Li vs. Kim Jiseok
$$ ---------------------------------------
$$ | . . . . O . . . O . . . O . . . . . . |
$$ | . . . . O X X X O . O X O X X O O . . |
$$ | . . X O O O X X O . O X O O O X O O . |
$$ | . . . O X O . . O O O O X X O X X X O |
$$ | . O . O X O X X . . X X X O O X . O O |
$$ | . . X O X X . X O O X O O X O X X O a |
$$ | . . . O X . X X X X . X . X X . . X O |
$$ | O . O O X X O O X X . X . O O X . X X |
$$ | X O O . X O X O O . X . X . O X . . X |
$$ | X . O O O O . O O X . . . X O X X X O |
$$ | X . X X O . O O O X X X X X O . . O O |
$$ | . X . O O X X X O O . . O X O . O . . |
$$ | . . X X O X . . X O X X O O O . . O X |
$$ | X X O X O O X X X O O X X O O X X X . |
$$ | O O O X X X . . X O O O O O X . . X . |
$$ | . O . O O X . . . X X X X O X X X . . |
$$ | X X O O . O X X . . . . . X X . . . . |
$$ | . O . O . O X . . . . . . . . . . . . |
$$ | X X O . O 1 O X . . . . . . . . . . . |
$$ ---------------------------------------

Click Here To Show Diagram Code

[go]$$Bc Move #241, Game 2, Gu Li vs. Kim Jiseok
$$ ---------------------------------------
$$ | . . . . O . . . O . . . O . . . . . . |
$$ | . . . . O X X X O . O X O X X O O . . |
$$ | . . X O O O X X O . O X O O O X O O . |
$$ | . . . O X O . . O O O O X X O X X X O |
$$ | . O . O X O X X . . X X X O O X . O O |
$$ | . . X O X X . X O O X O O X O X X O a |
$$ | . . . O X . X X X X . X . X X . . X O |
$$ | O . O O X X O O X X . X . O O X . X X |
$$ | X O O . X O X O O . X . X . O X . . X |
$$ | X . O O O O . O O X . . . X O X X X O |
$$ | X . X X O . O O O X X X X X O . . O O |
$$ | . X . O O X X X O O . . O X O . O . . |
$$ | . . X X O X . . X O X X O O O . . O X |
$$ | X X O X O O X X X O O X X O O X X X . |
$$ | O O O X X X . . X O O O O O X . . X . |
$$ | . O . O O X . . . X X X X O X X X . . |
$$ | X X O O . O X X . . . . . X X . . . . |
$$ | . O . O . O X . . . . . . . . . . . . |
$$ | X X O . O 1 O X . . . . . . . . . . . |
$$ ---------------------------------------[/go]

The original question was: Why did Black and White fight over this ko (F1) when there is another ko at 'a' (T14) that allows them to split the two kos evenly?

[illluck]

If you can read Chinese, the 17th post is probably right (the poster is apparently a pro).

"The gist is that there are two half-point ko, black has Q8 for one point (bQ8, wQ7, bR7, wQ9, bS8, wr9, bT8 - go to link and the diagram is on the second page), and F9 is also a point.

After white took the ko, if black plays Q8, then white doesn't need the point at F9 anymore, and can fill in one of the ko. After that the game depends on the last ko.

If black fights ko and white takes the point at F9, then black will take the point at Q8 and then win because each side will get one of the remaining ko.

If both sides first fill in one ko each, then white will play on the right side and get a ko in sente. After that white will take the point at F9, then the game once again depends on a ko (the new one on the right side)."

I have no idea if she is right, but her analysis seems to make sense. Endgame is really incredibly complex!

Edit: Opps, just realized this is probably a discussion question - I went ahead and hid the translation because I spent a bit of time on it and didn't want to just delete it XD

[hiyayang]

If you can read Chinese, the 17th post is probably right (the poster is apparently a pro).

"The gist is that there are two half-point ko, black has Q8 for one point (bQ8, wQ7, bR7, wQ9, bS8, wr9, bT8 - go to link and the diagram is on the second page), and F9 is also a point.

After white took the ko, if black plays Q8, then white doesn't need the point at F9 anymore, and can fill in one of the ko. After that the game depends on the last ko.

If black fights ko and white takes the point at F9, then black will take the point at Q8 and then win because each side will get one of the remaining ko.

If both sides first fill in one ko each, then white will play on the right side and get a ko in sente. After that white will take the point at F9, then the game once again depends on a ko (the new one on the right side)."

I have no idea if she is right, but her analysis seems to make sense. Endgame is really incredibly complex!

Edit: Opps, just realized this is probably a discussion question - I went ahead and hid the translation because I spent a bit of time on it and didn't want to just delete it XD

That answered most part of the question, but it glossed over some important detail, which might be obvious to pros, but not so obvious to us amateurs. To give you something to think about, why couldn't W fill T14, allowing B to fill G1, and then proceed as the last part of the answer said "If both sides first fill in one ko each, ..."?

[ndkrempel]

One possible reason is that whoever gets to fill the G1 ko gets extra ko threats in the upcoming ko fight on the right hand side:

$$Bc Move #241, Game 2, Gu Li vs. Kim Jiseok
$$ ---------------------------------------
$$ | . . . . O . . . O . . . O . . . . . . |
$$ | . . . . O X X X O . O X O X X O O . . |
$$ | . . X O O O X X O . O X O O O X O O . |
$$ | . . . O X O . . O O O O X X O X X X O |
$$ | . O . O X O X X . . X X X O O X . O O |
$$ | . . X O X X . X O O X O O X O X X O 2 |
$$ | . . . O X . X X X X . X . X X . . X O |
$$ | O . O O X X O O X X . X . O O X . X X |
$$ | X O O . X O X O O . X . X . O X . . X |
$$ | X . O O O O . O O X . . . X O X X X O |
$$ | X . X X O . O O O X X X X X O . 4 O O |
$$ | . X . O O X X X O O . . O X O . O . 6 |
$$ | . . X X O X . . X O X X O O O . 5 O X |
$$ | X X O X O O X X X O O X X O O X X X . |
$$ | O O O X X X . . X O O O O O X . . X . |
$$ | . O . O O X . . . X X X X O X X X . . |
$$ | X X O O . O X X . . . . . X X . . . . |
$$ | . O . O . O X . . . . . . . . . . . . |
$$ | X X O . O 1 3 X . . . . . . . . . . . |
$$ ---------------------------------------

Click Here To Show Diagram Code

[go]$$Bc Move #241, Game 2, Gu Li vs. Kim Jiseok
$$ ---------------------------------------
$$ | . . . . O . . . O . . . O . . . . . . |
$$ | . . . . O X X X O . O X O X X O O . . |
$$ | . . X O O O X X O . O X O O O X O O . |
$$ | . . . O X O . . O O O O X X O X X X O |
$$ | . O . O X O X X . . X X X O O X . O O |
$$ | . . X O X X . X O O X O O X O X X O 2 |
$$ | . . . O X . X X X X . X . X X . . X O |
$$ | O . O O X X O O X X . X . O O X . X X |
$$ | X O O . X O X O O . X . X . O X . . X |
$$ | X . O O O O . O O X . . . X O X X X O |
$$ | X . X X O . O O O X X X X X O . 4 O O |
$$ | . X . O O X X X O O . . O X O . O . 6 |
$$ | . . X X O X . . X O X X O O O . 5 O X |
$$ | X X O X O O X X X O O X X O O X X X . |
$$ | O O O X X X . . X O O O O O X . . X . |
$$ | . O . O O X . . . X X X X O X X X . . |
$$ | X X O O . O X X . . . . . X X . . . . |
$$ | . O . O . O X . . . . . . . . . . . . |
$$ | X X O . O 1 3 X . . . . . . . . . . . |
$$ ---------------------------------------[/go]

Note that this ko fight is essentially about getting sente to play F9. Also note that black can afford to back down by connecting at T6, white F9, then black can still fight the 1/2-point ko at S8, and now has E2 as a ko threat. Conversely, white doesn't have ko threats at H2 and J1 to use.

[Bill Spight]

Well, I think I understand this, at least in a technical sense. I did not read out the whole ko fight.

(;CA[ISO8859-1]ST[2]FF[4]SZ[19]AP[GOWrite:2.2.21]GM[1]GN[Move #241, Game 2, Gu Li vs. Kim Jiseok]LB[sf:a]FG[259:]PM[0]AB[mi][pi][ki][si][gl][fl][hl][bl][ei][gi][ai][nf][ed][hf][kf][md][nl][pf][qf][nd][qd][pd][nq][oq][bq][aq][hq][gq][eg][ig][jg][gg][hg][ng][lg][rg][og][rd][hs][bs][as][qn][pn][hb][rn][fb][gb][hn][in][ob][ln][nb][mn][lb][an][bn][dn][gn][ge][he][im][ee][lm][km][ke][fm][le][sm][jp][rj][kp][lp][pj][mp][qj][fp][nj][jj][op][pp][qp][aj][me][pe][cc][jk][kk][ro][lk][oo][mk][nk][ak][ck][dk][pc][fo][lc][eo][do][gc][gr][hc][io][fh][ef][ih][ff][jh][lh][cf][eh][ph][rh][cm][sh][dm]DT[2010-11-05]AW[oi][ea][ii][ia][ma][il][el][dl][fi][hi][bi][ci][lf][mf][of][dd][if][fd][jf][id][kd][jd][ql][ld][ol][od][ml][rf][jl][dq][cq][fq][dg][ok][rk][sk][sg][sd][gs][es][cs][kb][ib][eb][qb][jn][pb][kn][mb][nn][on][cn][en][fn][jm][fe][em][rm][nm][de][mm][be][om][sj][oj][hj][ij][np][fj][dj][ej][cj][ne][bp][ep][dp][oe][re][se][mo][dc][gk][no][hk][fc][ik][ko][ec][lo][ek][br][oc][bo][ao][qc][rc][kc][mc][nc][co][fr][jo][dr][ic][gh][df][hh][ah][ch][dh][nh][oh]PL[B]PW[ ]C[Move #241, Game 2, Gu Li vs. Kim Jiseok]PC[ ]PB[ ]
;MN[1]PM[2]C[*** White can now make a mistake.]B[fs]
(
;W[fk]C[*** This gains 1/2 point locally, but is a mistake.]
;B[pl]
;W[pm]
;C[*** Sacrificing the stone.]B[qm]
;W[pk]
;B[rl]
;W[qk]
;C[This sequence gains 2/3 point, but only after White has made the previous play, because it gives White an eye, which makes White's previous move unnecessary. The two kos cancel out.]B[sl]
)
(
;W[sl]C[*** Not an easy play to find, perhaps. And not right at this time, I think.]
;B[sn]
;W[qk]
;B[qm]
;W[fk]C[*** Now is the time, at least in this region.]
;C[*** However, by winning this ko, Black gains a ko threat. That is the technical reason for fighting this ko first. And, as ndkrempel points out, if White wins this ko White gets ko threats.]B[gs]
)

)

How about them apples?

[hiyayang]

Yes, ko threats was the reason, which is why when B invoked the ko threat @ B16, W had no choice but to fill F1, gambling on B16 being no threat. By the way, winning the ko fight @ F1 gave W three additional ko threats, @ H4 (after W G4, B G5 exchange), H2 and J1.