Difference between revisions of "2006 AMC 12B Problems/Problem 25"
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<cmath>t \mapsto 999 - t</cmath> | <cmath>t \mapsto 999 - t</cmath> | ||
is a 1-1 correspondence between the odd and even numbers less than and relatively prime to <math>999</math>. So our final answer is <math>648/2 = 324</math>, or <math>\boxed{\text{B}}</math>. | is a 1-1 correspondence between the odd and even numbers less than and relatively prime to <math>999</math>. So our final answer is <math>648/2 = 324</math>, or <math>\boxed{\text{B}}</math>. | ||
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+ | ==Solution 2 (estimation)== | ||
+ | As in Solution 1, we show that <math>a_2</math> must be odd. We notice that, after experimentation, all numbers eventually end up in a sequence <math>1,1,0,1,1,0,\ldots</math>, which points to the assumption that every <math>2</math> out of <math>3</math> values for <math>a_2</math> work due to residue classes. Combining this with an estimated 50-50 chance that these values are either odd or even, since <math>999</math> values are nonnegative integers less than <math>999</math>, we can estimate that there are about <math>999\cdot\frac{2}{3}\cdot\frac{1}{2}=333</math> different values of <math>a_2</math> that work. The closest answer choice by far is <math>\boxed{\textbf{(B) }324}</math>. | ||
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+ | ~eevee9406 | ||
== See also == | == See also == | ||
{{AMC12 box|year=2006|ab=B|num-b=24|after=Last Question}} | {{AMC12 box|year=2006|ab=B|num-b=24|after=Last Question}} | ||
{{MAA Notice}} | {{MAA Notice}} |
Latest revision as of 22:32, 9 November 2024
Contents
[hide]Problem
A sequence of non-negative integers is defined by the rule for . If , and , how many different values of are possible?
Solution 1
We say the sequence completes at if is the minimal positive integer such that . Otherwise, we say does not complete.
Note that if , then for all , and does not divide , so if , then does not complete. (Also, cannot be 1 in this case since does not divide , so we do not care about these at all.)
From now on, suppose .
We will now show that completes at for some . We will do this with 3 lemmas.
Lemma: If , and neither value is , then .
Proof: There are 2 cases to consider.
If , then , and . So and .
If , then , and . So and .
In both cases, , as desired.
Lemma: If , then . Moreover, if instead we have for some , then .
Proof: By the way is constructed in the problem statement, having two equal consecutive terms implies that divides every term in the sequence. So and , so , so . For the proof of the second result, note that if , then , so by the first result we just proved, .
Lemma: completes at for some .
Proof: Suppose completed at some or not at all. Then by the second lemma and the fact that neither nor are , none of the pairs can have a or be equal to . So the first lemma implies so , a contradiction. Hence completes at for some .
Now we're ready to find exactly which values of we want to count.
Let's keep in mind that and that is odd. We have two cases to consider.
Case 1: If is odd, then is even, so is odd, so is odd, so is even, and this pattern must repeat every three terms because of the recursive definition of , so the terms of reduced modulo 2 are
so is odd and hence (since if completes at , then must be or for all ).
Case 2: If is even, then is odd, so is odd, so is even, so is odd, and this pattern must repeat every three terms, so the terms of reduced modulo 2 are so is even, and hence .
We have found that is true precisely when and is odd. This tells us what we need to count.
There are numbers less than and relatively prime to it ( is the Euler totient function). We want to count how many of these are odd. Note that
is a 1-1 correspondence between the odd and even numbers less than and relatively prime to . So our final answer is , or .
Solution 2 (estimation)
As in Solution 1, we show that must be odd. We notice that, after experimentation, all numbers eventually end up in a sequence , which points to the assumption that every out of values for work due to residue classes. Combining this with an estimated 50-50 chance that these values are either odd or even, since values are nonnegative integers less than , we can estimate that there are about different values of that work. The closest answer choice by far is .
~eevee9406
See also
2006 AMC 12B (Problems • Answer Key • Resources) | |
Preceded by Problem 24 |
Followed by Last Question |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25 | |
All AMC 12 Problems and Solutions |
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