MathDB
Problems
Contests
International Contests
IMO Shortlist
2022 IMO Shortlist
A5
A5
Part of
2022 IMO Shortlist
Problems
(1)
Pairwise differences form geometric sequence
Source: ISL 2022 A5
7/9/2023
Find all positive integers
n
⩾
2
n \geqslant 2
n
⩾
2
for which there exist
n
n
n
real numbers
a
1
<
⋯
<
a
n
a_1<\cdots<a_n
a
1
<
⋯
<
a
n
and a real number
r
>
0
r>0
r
>
0
such that the
1
2
n
(
n
−
1
)
\tfrac{1}{2}n(n-1)
2
1
n
(
n
−
1
)
differences
a
j
−
a
i
a_j-a_i
a
j
−
a
i
for
1
⩽
i
<
j
⩽
n
1 \leqslant i<j \leqslant n
1
⩽
i
<
j
⩽
n
are equal, in some order, to the numbers
r
1
,
r
2
,
…
,
r
1
2
n
(
n
−
1
)
r^1,r^2,\ldots,r^{\frac{1}{2}n(n-1)}
r
1
,
r
2
,
…
,
r
2
1
n
(
n
−
1
)
.
algebra
geometric sequence