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National and Regional Contests
France Contests
French Mathematical Olympiad
1993 French Mathematical Olympiad
Problem 2
Problem 2
Part of
1993 French Mathematical Olympiad
Problems
(1)
existence of increasing sequences
Source: France 1993 P2
5/12/2021
Let
n
n
n
be a given positive integer.(a) Do there exist
2
n
+
1
2n+1
2
n
+
1
consecutive positive integers
a
0
,
a
1
,
…
,
a
2
n
a_0,a_1,\ldots,a_{2n}
a
0
,
a
1
,
…
,
a
2
n
in the ascending order such that
a
0
+
a
1
+
…
+
a
n
=
a
n
+
1
+
…
+
a
2
n
a_0+a_1+\ldots+a_n=a_{n+1}+\ldots+a_{2n}
a
0
+
a
1
+
…
+
a
n
=
a
n
+
1
+
…
+
a
2
n
? (b) Do there exist consecutive positive integers
a
0
,
a
+
1
,
…
,
a
2
n
a_0,a+1,\ldots,a_{2n}
a
0
,
a
+
1
,
…
,
a
2
n
in ascending order such that
a
0
2
+
a
1
2
+
…
+
a
n
2
=
a
n
+
1
2
+
…
+
a
2
n
2
a_0^2+a_1^2+\ldots+a_n^2=a_{n+1}^2+\ldots+a_{2n}^2
a
0
2
+
a
1
2
+
…
+
a
n
2
=
a
n
+
1
2
+
…
+
a
2
n
2
? (c) Do there exist consecutive positive integers
a
0
,
a
1
,
…
,
a
2
n
a_0,a_1,\ldots,a_{2n}
a
0
,
a
1
,
…
,
a
2
n
in ascending order such that
a
0
3
+
a
1
3
+
…
+
a
n
3
=
a
n
+
1
3
+
…
+
a
2
n
3
a_0^3+a_1^3+\ldots+a_n^3=a_{n+1}^3+\ldots+a_{2n}^3
a
0
3
+
a
1
3
+
…
+
a
n
3
=
a
n
+
1
3
+
…
+
a
2
n
3
?[hide=Official Hint]You may study the function
f
(
x
)
=
(
x
−
n
)
3
+
…
+
x
3
−
(
x
+
1
)
3
−
…
−
(
x
+
n
)
3
f(x)=(x-n)^3+\ldots+x^3-(x+1)^3-\ldots-(x+n)^3
f
(
x
)
=
(
x
−
n
)
3
+
…
+
x
3
−
(
x
+
1
)
3
−
…
−
(
x
+
n
)
3
and prove that the equation
f
(
x
)
=
0
f(x)=0
f
(
x
)
=
0
has a unique solution
x
n
x_n
x
n
with
3
n
(
n
+
1
)
<
x
n
<
3
n
(
n
+
1
)
+
1
3n(n+1)<x_n<3n(n+1)+1
3
n
(
n
+
1
)
<
x
n
<
3
n
(
n
+
1
)
+
1
. You may use the identity
1
3
+
2
3
+
…
+
n
3
=
n
2
(
n
+
1
)
2
2
1^3+2^3+\ldots+n^3=\frac{n^2(n+1)^2}2
1
3
+
2
3
+
…
+
n
3
=
2
n
2
(
n
+
1
)
2
.
number theory
Sequences