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Putnam
2023 Putnam
A4
2023 Putnam A4
2023 Putnam A4
Source:
December 3, 2023
Putnam
Putnam 2023
Problem Statement
Let
v
1
,
…
,
v
12
v_1, \ldots, v_{12}
v
1
,
…
,
v
12
be unit vectors in
R
3
\mathbb{R}^3
R
3
from the origin to the vertices of a regular icosahedron. Show that for every vector
v
∈
R
3
v \in \mathbb{R}^3
v
∈
R
3
and every
ε
>
0
\varepsilon>0
ε
>
0
, there exist integers
a
1
,
…
,
a
12
a_1, \ldots, a_{12}
a
1
,
…
,
a
12
such that
∥
a
1
v
1
+
⋯
+
a
12
v
12
−
v
∥
<
ε
\left\|a_1 v_1+\cdots+a_{12} v_{12}-v\right\|<\varepsilon
∥
a
1
v
1
+
⋯
+
a
12
v
12
−
v
∥
<
ε
.
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