(;GM[1]FF[4]CA[UTF-8]AP[CGoban:3]ST[2]
RU[Japanese]SZ[19]KM[0.00]

;B[pd]C[Let's start with a professional game between Baek Hong Suk 2p playing black and Yoo Jae Hyeong 6p playing white.]
(;AB[mc][qg]C[How to live in this corner.
It's easy to make a ko. But what if you want to live without ko and that is enough to win?
There is certain way to live in the corner for White.]
(;W[qc]
;B[qd]
(;W[pc]C[This move is better.]
;B[oc]
;W[ob]
;B[nb]
;W[od]
;B[nc]
;W[rd]
;B[re]
;W[rb]
;B[sd]C[Here is the real problem.
Can you live without ko for White?
]
(;W[sc]C[This would be easiest way to make a ko for White. In professional games you always see this move. But it is good to know you can also live without a ko.
])
(;W[pa]C[This is the answer.]
(;B[rc]
;W[sb]C[Be careful though if White is losing after this simple variation it might have been better to make a ko.
It depends on the whole board. Whether White has more ko threats and if he is winning or losing.]
;B[na]
;W[ra])
(;B[sb]C[This seems to work for Black but actually it doesn't.]
;W[rc]
;B[sc]
;W[ra]
;B[qb]
;W[sa]
;B[se]
;W[qe]
;B[pe]
;W[qf]C[It is a disaster for Black.])
(;B[na]C[This extension is tricky.]
(;W[qa]C[This one is the worst.]
;B[rc]LB[pb:B][sb:A]C[A or B. White dead.])
(;W[rc]
;B[pb]
;W[qb]
;B[oa]LB[sc:A]C[It's ko again When white plays at A. But white has a better move])
(;W[ra]C[This one is good. White can still live.]
(;B[sb]
;W[rc]
;B[sc]
;W[sa]C[It comes back to the previous variation.])
(;B[oa]C[Black plays atari to remove White's one liberty.]
;W[pb]
;B[sb]
;W[rc]
;B[sc]
(;W[sa]C[If White plays atari first it won't work.]
;B[se]
;W[qe]
;B[pe]
;W[qf]
;B[qb]C[See the difference?
])
(;W[qe]C[But White can cut here first.]
;B[pe]
;W[qf]
;B[rf]
;W[sa]C[Then atari.]
;B[se]
;W[pf]C[Now it works.]))
(;B[pb]C[But what if Black threw in here first in order to remove one more liberty.]
;W[qb]
;B[oa]C[Then how can White play?]
(;W[pb]C[This connection is too innocent.]
;B[sb]
;W[rc]
;B[sc]
;W[qe]
;B[pe]
;W[qf]
;B[rf]
;W[pf]
;B[oe]LB[sa:A]C[Now White can't play at A. White is dead.])
(;W[sc]C[This is the answer.]
;B[rc]
;W[sb]C[What do you think?
It is double ko.]
;B[pb]
;W[rd]
;B[se]
;W[ob]
;B[ei]
;W[fi]
;B[pb]
;W[qe]
;B[pe]
;W[qf]C[In reality, this ko would not be double ko. Because it can give really big damage to Black.]
;B[rf]
;W[pf]
;B[oe]
;W[rg]LB[ob:B][rc:A]C[Even if black win the ko at A, the corner is still ko at B.
So, White will get a lot of compensation for the two kos or live in the corner.]))))))
(;W[pb]C[This move might be okay for White to live. It depends on how strong the outside of Black is.]
(;B[rc]C[Playing here is only good when black is very strong outside.]
;W[rb]
;B[pc]
;W[qb]
;B[rd]
;W[nb]TR[kc]C[If there is a Black stone at the triangle, White is dead, see variation.]
;B[mb]C[If black can block here due to strength outside, Black can kill White.]
;W[sb]
;B[ra]
;W[nc]C[Basically White is alive.]
;B[oc]
;W[ob]
;B[na]
;W[oa]
;B[qa]C[Black can go for the kill when he is very strong on the outside. In this case it would be bad])
(;B[ob]C[this move is not that good either.]
;W[oc]
;B[nb]
;W[pc]
;B[ne]
;W[rb]C[White can live with this move.]
(;B[rd]
;W[qa]C[Alive.])
(;B[qa]C[This does not kill]
;W[rd]
;B[re]
;W[sc]C[White can live]))
(;B[nb]C[This response is more complicated.]
(;W[rb]
;B[rd]C[White is in danger.]
(;W[rc]
;B[qa]
;W[ob]
;B[sc]C[There is no way out for White.])
(;W[qa]C[This move is a bit better for White.]
;B[rc]
;W[pc]
;B[sb]
;W[ob]
;B[oa]
;W[od]
;B[ne]C[It may depend on outside but it seems very dangerous for White.]
;W[oe]
;B[of]
;W[pf]
;B[nd]
;W[pe]
;B[pg]C[In this case White is dead.]))
(;W[rd]C[White can make a ko.]
;B[re]
;W[rb]
;B[sd]
;W[sc]C[But there is a better way to make ko.]))))
(;B[kc]C[When black has a stone here the situation changes]
;W[qc]
;B[qd]
;W[pb]
;B[rc]
;W[rb]
;B[pc]
;W[qb]
;B[rd]
;W[nb]
;B[oc]
;W[ob]
;B[sb]C[White dies]))
(;W[dd]
;B[pq]
;W[dp]
;B[fc]
;W[cf]
;B[mc]
;W[po]
;B[qo]
;W[qn]
;B[qp]
;W[pm]
;B[nq]
;W[qi]
;B[qg]LB[qc:A][ql:B]SQ[mc]C[Normally this move is not recommended when you already have played an ogeima at the marked stone. The corner is still wide open at A. In this case black plays here looking forward to invade at B. And white defended in the game.]
;W[oi]C[Now black played tenuki]
;B[cn]LB[qg:A]C[Black played A to strenghten his corner a bit in sente. But let's see what is still possible when white invades!]))
