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Difference between revisions of "1973 USAMO Problems/Problem 2"

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<center> <math>X_0=1</math>, <math>X_1=1</math>, <math>X_{n+1}=X_n+2X_{n-1}</math> <math>(n=1,2,3,\dots),</math></center>
 
<center> <math>X_0=1</math>, <math>X_1=1</math>, <math>X_{n+1}=X_n+2X_{n-1}</math> <math>(n=1,2,3,\dots),</math></center>
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<center><math>Y_0=1</math>, <math>Y_1=1</math>, <math>Y_{n+1}=2Y_n+3Y_{n-1}</math> <math>(n=1,2,3,\dots)</math>.</center>
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<center><math>Y_0=1</math>, <math>Y_1=7</math>, <math>Y_{n+1}=2Y_n+3Y_{n-1}</math> <math>(n=1,2,3,\dots)</math>.</center>
  
 
Thus, the first few terms of the sequences are:
 
Thus, the first few terms of the sequences are:
  
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<center> <math>1</math>, <math>1</math>, <math>3</math>, <math>5</math>, <math>11</math>, <math>21</math>, <math>\dots</math>,</center>
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<center> <math>X:1, 1, 3, 5, 11, 21, \dots</math>,</center>
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<center> <math>1</math>, <math>7</math>, <math>17</math>, <math>55</math>, <math>161</math>, <math>487</math>, <math>\dots</math>.</center>
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<center> <math>Y:1, 7, 17, 55, 161, 487, \dots</math>.</center>
  
 
Prove that, except for the "1", there is no term which occurs in both sequences.
 
Prove that, except for the "1", there is no term which occurs in both sequences.
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Combining both results, we see that <math>X_i</math> and <math>Y_j</math> are not congruent <math>\bmod{8}</math> when <math>i\geq 3</math> and <math>j\geq 2</math>. Thus after the "1", the terms of each sequence are not equal.
 
Combining both results, we see that <math>X_i</math> and <math>Y_j</math> are not congruent <math>\bmod{8}</math> when <math>i\geq 3</math> and <math>j\geq 2</math>. Thus after the "1", the terms of each sequence are not equal.
  
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==See also==
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{{alternate solutions}}
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==See Also==
  
 
{{USAMO box|year=1973|num-b=1|num-a=3}}
 
{{USAMO box|year=1973|num-b=1|num-a=3}}
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{{MAA Notice}}
  
 
[[Category:Olympiad Number Theory Problems]]
 
[[Category:Olympiad Number Theory Problems]]

Latest revision as of 18:54, 3 July 2013

Problem

Let $\{X_n\}$ and $\{Y_n\}$ denote two sequences of integers defined as follows:

$X_0=1$, $X_1=1$, $X_{n+1}=X_n+2X_{n-1}$ $(n=1,2,3,\dots),$
$Y_0=1$, $Y_1=7$, $Y_{n+1}=2Y_n+3Y_{n-1}$ $(n=1,2,3,\dots)$.

Thus, the first few terms of the sequences are:

$X:1, 1, 3, 5, 11, 21, \dots$,
$Y:1, 7, 17, 55, 161, 487, \dots$.

Prove that, except for the "1", there is no term which occurs in both sequences.

Solution

We can look at each sequence $\bmod{8}$:

$X$: $1$, $1$, $3$, $5$, $3$, $5$, $\dots$,
$Y$: $1$, $7$, $1$, $7$, $1$, $7$, $\dots$.
  • Proof that $X$ repeats $\bmod{8}$:

The third and fourth terms are $3$ and $5$ $\bmod{8}$. Plugging into the formula, we see that the next term is $11\equiv 3\bmod{8}$, and plugging $5$ and $3$, we get that the next term is $13\equiv 5\bmod{8}$. Thus the sequence $X$ repeats, and the pattern is $3,5,3,5,\dots$.

  • Proof that $Y$ repeats $\bmod{8}$:

The first and second terms are $1$ and $7$ $\bmod{8}$. Plugging into the formula, we see that the next term is $17\equiv 1\bmod{8}$, and plugging $7$ and $1$, we get that the next term is $23\equiv 7\bmod{8}$. Thus the sequence $Y$ repeats, and the pattern is $1,7,1,7,\dots$.


Combining both results, we see that $X_i$ and $Y_j$ are not congruent $\bmod{8}$ when $i\geq 3$ and $j\geq 2$. Thus after the "1", the terms of each sequence are not equal.



Alternate solutions are always welcome. If you have a different, elegant solution to this problem, please add it to this page.

See Also

1973 USAMO (Problems • Resources)
Preceded by
Problem 1 		Followed by
Problem 3
1 • 2 • 3 • 4 • 5
All USAMO Problems and Solutions

The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions. AMC logo.png

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