(;GM[1]FF[4]CA[UTF-8]AP[CGoban:3]ST[2]
RU[Japanese]SZ[19]KM[0.00]
C[Getting stronger in the corner 2]PL[B]
(;B[pd]
;B[mc]
;B[qe]LB[ob:C][oc:B][qc:A]C[Let's start with basic invasion.
There are three options normally (A-C).]
(;W[qc]LB[pc:A]C[After 3-3 invasion, A is normal answer. ]
(;B[pc]
(;W[rd]
(;B[re]
;W[pb]
(;B[ob]
;W[rb]
;B[pa]
;W[qa]C[ko
])
(;B[rb]
(;W[qb]
;B[ob]
;W[ra]
;B[sb]
;W[se]
(;B[sf]
;W[sd]
;B[pa]LB[rf:A]C[aji at A])
(;B[rg]
;W[pa]
;B[rc]
;W[qd]
;B[sd]C[ko but bad for black]))
(;W[oc]C[another aji ])))
(;B[qb]
;W[re]
;B[rf]
;W[qf]
;B[rg]
;W[qd]
;B[pe]
;W[rb]
;B[pb]
;W[ra]C[white is alive])
(;B[pb]
;W[re]
;B[rf]
;W[qb]
;B[se]LB[qf:A]C[Black can kill but big weak point at A is made which white can aim at]))
(;W[pb]
;B[ob]
(;W[rb]
;B[rd]C[captured])
(;W[rd]
(;B[re]
;W[rb]
;B[pa]
;W[qa]C[ko])
(;B[rb]
;W[re]
;B[rf]
;W[qb]
;B[ra]LB[oc:B][qf:A]C[two weak points are made])))
(;W[re]
(;B[rf]
(;W[rd]
;B[pb]C[same as before])
(;W[qf]
(;B[rd]
;W[pe]
(;B[qd]
;W[rg]
;B[se]C[good exchange for white
])
(;B[se]
;W[qd]
;B[re]
;W[oe]
;B[nd]
;W[qg]C[one possible sequence]))
(;B[pf]
;W[qd]
;B[qg]C[this ponuki is too big for black]
;W[pb]
;B[ob]
;W[rb]
;B[pa]
;W[qa]C[ko])))
(;B[rd]
;W[rf]
;B[rc]C[normal])))
(;B[rd]
;W[oc]C[hard to capture]))
(;W[oc]
(;B[od]C[not normal answer]
(;W[nc]
;B[md]
(;W[rb]
;B[ob]
;W[nb]
;B[qb]C[good sequence])
(;W[qc]
;B[rb]C[vital point])
(;W[pc]
;B[qc]
;W[qb]
;B[rc]
;W[mb]
;B[lb]
;W[ma]C[hard to kill]))
(;W[pc]
;B[qc]
;W[qb]
;B[rb]
;W[rc]
;B[qd]
;W[ra]
(;B[sb]
;W[ob]C[white is alive])
(;B[pb]
;W[sb]
;B[ob]C[live in sente])))
(;B[pc]C[normal answer]
(;W[pb]
;B[qb]
;W[od]
;B[oe]
(;W[ne]
;B[nd]
;W[of]
;B[pe]
;W[me]C[sacrifice stones])
(;W[kc]TR[kc]C[when white has a stone here]
(;W[md]C[possible])
(;W[nb]C[possible])))
(;W[me]
;B[nd]C[when white reduce black influence, possible])))
(;W[ob]LB[jc:A][kc:B]C[this move is played when white is at A or B.]
;W[jc]
(;B[oc]
(;W[nb]
;B[pb]
;W[nc]
;B[nd]
;W[mb]C[normal])
(;W[pb]
;B[nb]
;W[qc]
;B[rd]C[captured]
(;W[rc]
;B[oa]
;W[ra]
;B[sb]
;W[sc]
;B[rb]
;W[pa]
;B[qb])
(;W[rb]
;B[qa]
;W[ra]
;B[sb]
;W[sc]
;B[rc]))
(;W[qc]
;B[nb]
;W[re]
;B[rd]
;W[qd]
;B[rf]
;W[pe]
;B[se]
;W[qf]
;B[re]
;W[pb]
;B[pc]C[not easy to live]))
(;B[nc]C[white can make ko]
;W[oc]
;B[od]
;W[rc]
(;B[qb]
;W[qc]
;B[pc]
;W[pb]
;B[rb]
(;W[ra]
;B[qa]
;W[sb]
;B[sa]
;W[sc]
;B[nb]C[ko. White takes first.])
(;W[sb]
;B[ra]
;W[qa]
;B[pa]
;W[oa]C[Same kind of ko. White also takes first.]))
(;B[rd]
;W[qb]
;B[nb]
;W[oa]
;B[qc]
;W[rb]C[live]))))
(;AW[hc][hd][hg][qj][om][pm][qm]AB[lc][oc][pd][qg]
;W[qc]C[Let's assume white has more ko threats.]
;B[qd]
;W[rd]
;B[re]
;W[pb]C[How will you answer?]
(;B[ob]
;W[rb]
;B[sd]
;W[sc]C[White has more ko threat.])
(;B[rb]
(;W[rc]
;B[ob]
;W[sb]
;B[ra]C[good for black])
(;W[ob]
(;B[nb]
;W[rc]
;B[sd]
;W[sb]
;B[ra]
;W[nc]
;B[nd]
;W[oa]
;B[qa]
;W[mc]
;B[mb]
;W[md]
;B[ld]
;W[me]C[Even if black capture the white group in the corner, he will pay a lot.])
(;B[rc]
;W[nc]
;B[nd]
;W[mc]
;B[md]
;W[lb]C[black lost many points])))
(;B[rc]
;W[rb]
;B[sd]
;W[ob]
;B[nb]
;W[pc]
;B[nc]
;W[sb]C[black can not kill, so sente is the next option]))
(;AW[gc][gf][if][kf][kj][nk][pk]AB[jc][oc][ld][pd][qg][kh][hi][ji]
;W[me]
;B[md]C[how can white play?]
;W[qe]C[good try]
;B[qd]
;W[rd]
;B[rc]
(;W[pe]C[this is heavy.]
;B[oe]
(;W[pf]C[bad shape]
;B[oh])
(;W[of]
;B[pf]
;W[ne]
;B[od]
;W[og]
;B[qi]C[possible to sacrifice stone but too big now]))
(;W[pg]C[light move]
(;B[qh]
;W[qf]C[black is cut])
(;B[qf]
;W[pe]
;B[pf]
;W[oe]
;B[re]
;W[rf]
;B[sd]
;W[qh]C[good for white])
(;B[pf]
(;W[qh])
(;W[qf]
;B[pe]
;W[qh]
;B[rg]
;W[rh]C[good for white]))
(;B[ph]
;W[qh]C[complicated, but white makes aji in the black's territory])))
(;AW[fc][gd][ge][hf][qj][ll][ol][qm]AB[jc][oc][pd][qg]
;W[re]C[Example from a professional game ]
;B[qe]
(;W[rf]C[normal ]
;B[rd]
;W[qf]
;B[pf]
;W[rg]C[but white thinks this is not enough because white position outside is very strong. ])
(;W[qc]C[unusual move]
(;B[rd]C[not good]
;W[pc]C[good move]
(;B[qd]
;W[ob]
;B[nb]
;W[nc]
;B[od]
;W[mb]C[too good for white])
(;B[od]
;W[rc]
;B[qd]
;W[ob]
;B[nb]
;W[oa]
;B[qa]
;W[ra]
;B[rb]
;W[sb]C[ko]))
(;B[rf]C[correct answer]
;W[qd]
(;B[rd]
;W[ob]
;B[nb]
;W[rc]
;B[se]
(;W[pb]
;B[oa]
;W[nc]C[weak point])
(;W[pc]
;B[od]
;W[oa]C[ko]))
(;B[ob]C[good move to kill ]
;W[pe]
;B[qf]
;W[sd]
;B[rb]C[white in the corner can't live]
;W[oe]
;B[lc]
;W[ld]
;B[mc]
;W[og]C[white get some in the center]
;B[pi]
;W[qh]
;B[ph]
;W[rh]
;B[pg]
;W[ni]LB[sf:A]C[ko aji at A])))))
