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Cocompact group action
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Past exam of the mathematics course of the University of Cambridge
/
2025
/
iii
/
Paper 133
/
4
/
b
/
Solution
Created
2026-09-24
Updated
2026-09-24
View more
Choose
y
n
∈
X
with
d
ϕ
(
y
n
)
→
0
. By the
cocompact group action
, there is
a
compact
set
K
⊆
X
whose translates cover
X
. Choose
γ
n
∈
Γ
such that
x
n
:=
γ
n
y
n
∈
K
,
ϕ
n
:=
γ
n
ϕ
γ
n
−
1
.
(1)
Isometric invariance gives
d
ϕ
n
(
x
n
)
=
d
(
γ
n
y
n
,
γ
n
ϕ
y
n
)
=
d
ϕ
(
y
n
)
⟶
0.
(2)
For
a
fixed basepoint
x
0
, compactness of
K
gives
C
=
max
x
∈
K
d
(
x
0
,
x
)
<
∞
, so
d
(
x
0
,
x
n
)
≤
C
.
Solved by
gpt-5
.
6
-sol high.
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