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National and Regional Contests
Bulgaria Contests
Bulgarian Winter Tournament
2024 Bulgarian Winter Tournament
11.3
11.3
Part of
2024 Bulgarian Winter Tournament
Problems
(1)
Can the nominator of this sum be squarefree?
Source: Bulgarian Winter Tournament 2024 11.3
1/28/2024
Let
q
>
3
q>3
q
>
3
be a rational number, such that
q
2
−
4
q^2-4
q
2
−
4
is a perfect square of a rational number. The sequence
a
0
,
a
1
,
…
a_0, a_1, \ldots
a
0
,
a
1
,
…
is defined by
a
0
=
2
,
a
1
=
q
,
a
i
+
1
=
q
a
i
−
a
i
−
1
a_0=2, a_1=q, a_{i+1}=qa_i-a_{i-1}
a
0
=
2
,
a
1
=
q
,
a
i
+
1
=
q
a
i
−
a
i
−
1
for all
i
≥
1
i \geq 1
i
≥
1
. Is it true that there exist a positive integer
n
n
n
and nonzero integers
b
0
,
b
1
,
…
,
b
n
b_0, b_1, \ldots, b_n
b
0
,
b
1
,
…
,
b
n
with sum zero, such that if
∑
i
=
0
n
a
i
b
i
=
A
B
\sum_{i=0}^{n} a_ib_i=\frac{A} {B}
∑
i
=
0
n
a
i
b
i
=
B
A
for
(
A
,
B
)
=
1
(A, B)=1
(
A
,
B
)
=
1
, then
A
A
A
is squarefree?
number theory