Calculate the probability with a finite arithmetic progression
$begingroup$
We have a finite arithmetic progression $ a_n $, where $ n geq 3 $ and its $rneq 0 $.
We draw three different numbers. We have to calculate the probability, that
these three numbers in the order of drawing will create another arithmetic progression.
My proposition :
$ Omega={n!over (n-3)!}$
$ mathbf{A}= {n choose 3} cdot 2$
But I think that my way of thinking about way of counting $ mathbf{A}$ is incorrect.
Any suggestions how can I count it? Thanks in advice, Kuba!
probability problem-solving arithmetic-progressions
$endgroup$
add a comment |
$begingroup$
We have a finite arithmetic progression $ a_n $, where $ n geq 3 $ and its $rneq 0 $.
We draw three different numbers. We have to calculate the probability, that
these three numbers in the order of drawing will create another arithmetic progression.
My proposition :
$ Omega={n!over (n-3)!}$
$ mathbf{A}= {n choose 3} cdot 2$
But I think that my way of thinking about way of counting $ mathbf{A}$ is incorrect.
Any suggestions how can I count it? Thanks in advice, Kuba!
probability problem-solving arithmetic-progressions
$endgroup$
2
$begingroup$
Is the progression finite?
$endgroup$
– EuxhenH
Jan 7 at 18:05
$begingroup$
I totally forgot about it :/. Yes, the progression is finite.
$endgroup$
– Kukoz
Jan 7 at 18:15
add a comment |
$begingroup$
We have a finite arithmetic progression $ a_n $, where $ n geq 3 $ and its $rneq 0 $.
We draw three different numbers. We have to calculate the probability, that
these three numbers in the order of drawing will create another arithmetic progression.
My proposition :
$ Omega={n!over (n-3)!}$
$ mathbf{A}= {n choose 3} cdot 2$
But I think that my way of thinking about way of counting $ mathbf{A}$ is incorrect.
Any suggestions how can I count it? Thanks in advice, Kuba!
probability problem-solving arithmetic-progressions
$endgroup$
We have a finite arithmetic progression $ a_n $, where $ n geq 3 $ and its $rneq 0 $.
We draw three different numbers. We have to calculate the probability, that
these three numbers in the order of drawing will create another arithmetic progression.
My proposition :
$ Omega={n!over (n-3)!}$
$ mathbf{A}= {n choose 3} cdot 2$
But I think that my way of thinking about way of counting $ mathbf{A}$ is incorrect.
Any suggestions how can I count it? Thanks in advice, Kuba!
probability problem-solving arithmetic-progressions
probability problem-solving arithmetic-progressions
edited Jan 7 at 19:14
Kukoz
asked Jan 7 at 17:28
KukozKukoz
409
409
2
$begingroup$
Is the progression finite?
$endgroup$
– EuxhenH
Jan 7 at 18:05
$begingroup$
I totally forgot about it :/. Yes, the progression is finite.
$endgroup$
– Kukoz
Jan 7 at 18:15
add a comment |
2
$begingroup$
Is the progression finite?
$endgroup$
– EuxhenH
Jan 7 at 18:05
$begingroup$
I totally forgot about it :/. Yes, the progression is finite.
$endgroup$
– Kukoz
Jan 7 at 18:15
2
2
$begingroup$
Is the progression finite?
$endgroup$
– EuxhenH
Jan 7 at 18:05
$begingroup$
Is the progression finite?
$endgroup$
– EuxhenH
Jan 7 at 18:05
$begingroup$
I totally forgot about it :/. Yes, the progression is finite.
$endgroup$
– Kukoz
Jan 7 at 18:15
$begingroup$
I totally forgot about it :/. Yes, the progression is finite.
$endgroup$
– Kukoz
Jan 7 at 18:15
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
$Omega = n(n-1)(n-2)$
We're interested in triples whose elements differ by $r$, $2r$ ... up to $lceil n/3rceil r$
Notice, that if a triple $a_i,a_k,a_l$ is OK, then so is $a_l,a_k,a_i$.
My idea was to "stick together" those desired elements that form each triple and calculate their quantity. To be precise: there are $2(n-2)$ triples whose elements differ by r, $2(n-4)$ triples whose elements differ by $2r$ .... and $2(n-2lceil n/3rceil)$ triples whose elements differ by $lceil n/3rceil r$. Then:
$P(A) = frac{2(n-2+n-4...+n-2 lceil n/3rceil)}{n(n-1)(n-2)}$
$endgroup$
1
$begingroup$
Not exactly we have different possibilities For example when the rest of $ {n 3} $ is $0$, your idea is good, -when the rest of $ {n 3} $ is $ 1 $ $mathbf{A}= 2[n-2+n-4...+n-2 lceil n/3rceil]+1$, when the rest of $ {n 3} $ is $ 2 $ $mathbf{A}= 2[n-2+n-4...+n-2 lceil n/3rceil]+2$, so this is completely good answer :) Cheers
$endgroup$
– Kukoz
Jan 10 at 14:41
1
$begingroup$
No. The ceiling function isn't the floor one. en.wikipedia.org/wiki/Floor_and_ceiling_functions <- look at the plot of the "ceiling function" on the right
$endgroup$
– The Cat
Jan 10 at 16:28
1
$begingroup$
$lceil 0.5rceil $=1
$endgroup$
– The Cat
Jan 10 at 16:29
1
$begingroup$
I think you just mistook those two together.
$endgroup$
– The Cat
Jan 10 at 16:30
1
$begingroup$
Yeah, now everything is good! Big thanks for you, and shoutout!
$endgroup$
– Kukoz
Jan 10 at 16:33
|
show 4 more comments
$begingroup$
Let the sequence be $a_1,a_2,a_3,...a_n$. Then $|Omega | = {nchoose 3} = {n(n-1)(n-2)over 6}$
and each good 3-sequence is uniquely defined by it's middle term and the difference $d$. So if middle term is $a_m$ then the difference can be $d,2d,...,(m-1)d$.
So if we have $n=2k$, then we have $$1+2+...+(k-1)+(k-1)+...+2+1 = k(k-1)$$
good 3-sequences. So the probability is $$P = {6k(k-1)over 4k(2k-1)(k-1)} = {3over 2(n-1)}$$
If $n=2k+1$ then we have $$1+2+...+(k-1)+k+(k-1)+...+2+1 = k(k-1)+k=k^2$$
good 3-sequences. So the probability is $$P = {6k^2over 2(2k+1)k(2k-1)} = {3(n-1)over 2n(n-2)}$$
$endgroup$
add a comment |
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2 Answers
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2 Answers
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$begingroup$
$Omega = n(n-1)(n-2)$
We're interested in triples whose elements differ by $r$, $2r$ ... up to $lceil n/3rceil r$
Notice, that if a triple $a_i,a_k,a_l$ is OK, then so is $a_l,a_k,a_i$.
My idea was to "stick together" those desired elements that form each triple and calculate their quantity. To be precise: there are $2(n-2)$ triples whose elements differ by r, $2(n-4)$ triples whose elements differ by $2r$ .... and $2(n-2lceil n/3rceil)$ triples whose elements differ by $lceil n/3rceil r$. Then:
$P(A) = frac{2(n-2+n-4...+n-2 lceil n/3rceil)}{n(n-1)(n-2)}$
$endgroup$
1
$begingroup$
Not exactly we have different possibilities For example when the rest of $ {n 3} $ is $0$, your idea is good, -when the rest of $ {n 3} $ is $ 1 $ $mathbf{A}= 2[n-2+n-4...+n-2 lceil n/3rceil]+1$, when the rest of $ {n 3} $ is $ 2 $ $mathbf{A}= 2[n-2+n-4...+n-2 lceil n/3rceil]+2$, so this is completely good answer :) Cheers
$endgroup$
– Kukoz
Jan 10 at 14:41
1
$begingroup$
No. The ceiling function isn't the floor one. en.wikipedia.org/wiki/Floor_and_ceiling_functions <- look at the plot of the "ceiling function" on the right
$endgroup$
– The Cat
Jan 10 at 16:28
1
$begingroup$
$lceil 0.5rceil $=1
$endgroup$
– The Cat
Jan 10 at 16:29
1
$begingroup$
I think you just mistook those two together.
$endgroup$
– The Cat
Jan 10 at 16:30
1
$begingroup$
Yeah, now everything is good! Big thanks for you, and shoutout!
$endgroup$
– Kukoz
Jan 10 at 16:33
|
show 4 more comments
$begingroup$
$Omega = n(n-1)(n-2)$
We're interested in triples whose elements differ by $r$, $2r$ ... up to $lceil n/3rceil r$
Notice, that if a triple $a_i,a_k,a_l$ is OK, then so is $a_l,a_k,a_i$.
My idea was to "stick together" those desired elements that form each triple and calculate their quantity. To be precise: there are $2(n-2)$ triples whose elements differ by r, $2(n-4)$ triples whose elements differ by $2r$ .... and $2(n-2lceil n/3rceil)$ triples whose elements differ by $lceil n/3rceil r$. Then:
$P(A) = frac{2(n-2+n-4...+n-2 lceil n/3rceil)}{n(n-1)(n-2)}$
$endgroup$
1
$begingroup$
Not exactly we have different possibilities For example when the rest of $ {n 3} $ is $0$, your idea is good, -when the rest of $ {n 3} $ is $ 1 $ $mathbf{A}= 2[n-2+n-4...+n-2 lceil n/3rceil]+1$, when the rest of $ {n 3} $ is $ 2 $ $mathbf{A}= 2[n-2+n-4...+n-2 lceil n/3rceil]+2$, so this is completely good answer :) Cheers
$endgroup$
– Kukoz
Jan 10 at 14:41
1
$begingroup$
No. The ceiling function isn't the floor one. en.wikipedia.org/wiki/Floor_and_ceiling_functions <- look at the plot of the "ceiling function" on the right
$endgroup$
– The Cat
Jan 10 at 16:28
1
$begingroup$
$lceil 0.5rceil $=1
$endgroup$
– The Cat
Jan 10 at 16:29
1
$begingroup$
I think you just mistook those two together.
$endgroup$
– The Cat
Jan 10 at 16:30
1
$begingroup$
Yeah, now everything is good! Big thanks for you, and shoutout!
$endgroup$
– Kukoz
Jan 10 at 16:33
|
show 4 more comments
$begingroup$
$Omega = n(n-1)(n-2)$
We're interested in triples whose elements differ by $r$, $2r$ ... up to $lceil n/3rceil r$
Notice, that if a triple $a_i,a_k,a_l$ is OK, then so is $a_l,a_k,a_i$.
My idea was to "stick together" those desired elements that form each triple and calculate their quantity. To be precise: there are $2(n-2)$ triples whose elements differ by r, $2(n-4)$ triples whose elements differ by $2r$ .... and $2(n-2lceil n/3rceil)$ triples whose elements differ by $lceil n/3rceil r$. Then:
$P(A) = frac{2(n-2+n-4...+n-2 lceil n/3rceil)}{n(n-1)(n-2)}$
$endgroup$
$Omega = n(n-1)(n-2)$
We're interested in triples whose elements differ by $r$, $2r$ ... up to $lceil n/3rceil r$
Notice, that if a triple $a_i,a_k,a_l$ is OK, then so is $a_l,a_k,a_i$.
My idea was to "stick together" those desired elements that form each triple and calculate their quantity. To be precise: there are $2(n-2)$ triples whose elements differ by r, $2(n-4)$ triples whose elements differ by $2r$ .... and $2(n-2lceil n/3rceil)$ triples whose elements differ by $lceil n/3rceil r$. Then:
$P(A) = frac{2(n-2+n-4...+n-2 lceil n/3rceil)}{n(n-1)(n-2)}$
edited Jan 10 at 18:33
answered Jan 7 at 19:35
The CatThe Cat
25112
25112
1
$begingroup$
Not exactly we have different possibilities For example when the rest of $ {n 3} $ is $0$, your idea is good, -when the rest of $ {n 3} $ is $ 1 $ $mathbf{A}= 2[n-2+n-4...+n-2 lceil n/3rceil]+1$, when the rest of $ {n 3} $ is $ 2 $ $mathbf{A}= 2[n-2+n-4...+n-2 lceil n/3rceil]+2$, so this is completely good answer :) Cheers
$endgroup$
– Kukoz
Jan 10 at 14:41
1
$begingroup$
No. The ceiling function isn't the floor one. en.wikipedia.org/wiki/Floor_and_ceiling_functions <- look at the plot of the "ceiling function" on the right
$endgroup$
– The Cat
Jan 10 at 16:28
1
$begingroup$
$lceil 0.5rceil $=1
$endgroup$
– The Cat
Jan 10 at 16:29
1
$begingroup$
I think you just mistook those two together.
$endgroup$
– The Cat
Jan 10 at 16:30
1
$begingroup$
Yeah, now everything is good! Big thanks for you, and shoutout!
$endgroup$
– Kukoz
Jan 10 at 16:33
|
show 4 more comments
1
$begingroup$
Not exactly we have different possibilities For example when the rest of $ {n 3} $ is $0$, your idea is good, -when the rest of $ {n 3} $ is $ 1 $ $mathbf{A}= 2[n-2+n-4...+n-2 lceil n/3rceil]+1$, when the rest of $ {n 3} $ is $ 2 $ $mathbf{A}= 2[n-2+n-4...+n-2 lceil n/3rceil]+2$, so this is completely good answer :) Cheers
$endgroup$
– Kukoz
Jan 10 at 14:41
1
$begingroup$
No. The ceiling function isn't the floor one. en.wikipedia.org/wiki/Floor_and_ceiling_functions <- look at the plot of the "ceiling function" on the right
$endgroup$
– The Cat
Jan 10 at 16:28
1
$begingroup$
$lceil 0.5rceil $=1
$endgroup$
– The Cat
Jan 10 at 16:29
1
$begingroup$
I think you just mistook those two together.
$endgroup$
– The Cat
Jan 10 at 16:30
1
$begingroup$
Yeah, now everything is good! Big thanks for you, and shoutout!
$endgroup$
– Kukoz
Jan 10 at 16:33
1
1
$begingroup$
Not exactly we have different possibilities For example when the rest of $ {n 3} $ is $0$, your idea is good, -when the rest of $ {n 3} $ is $ 1 $ $mathbf{A}= 2[n-2+n-4...+n-2 lceil n/3rceil]+1$, when the rest of $ {n 3} $ is $ 2 $ $mathbf{A}= 2[n-2+n-4...+n-2 lceil n/3rceil]+2$, so this is completely good answer :) Cheers
$endgroup$
– Kukoz
Jan 10 at 14:41
$begingroup$
Not exactly we have different possibilities For example when the rest of $ {n 3} $ is $0$, your idea is good, -when the rest of $ {n 3} $ is $ 1 $ $mathbf{A}= 2[n-2+n-4...+n-2 lceil n/3rceil]+1$, when the rest of $ {n 3} $ is $ 2 $ $mathbf{A}= 2[n-2+n-4...+n-2 lceil n/3rceil]+2$, so this is completely good answer :) Cheers
$endgroup$
– Kukoz
Jan 10 at 14:41
1
1
$begingroup$
No. The ceiling function isn't the floor one. en.wikipedia.org/wiki/Floor_and_ceiling_functions <- look at the plot of the "ceiling function" on the right
$endgroup$
– The Cat
Jan 10 at 16:28
$begingroup$
No. The ceiling function isn't the floor one. en.wikipedia.org/wiki/Floor_and_ceiling_functions <- look at the plot of the "ceiling function" on the right
$endgroup$
– The Cat
Jan 10 at 16:28
1
1
$begingroup$
$lceil 0.5rceil $=1
$endgroup$
– The Cat
Jan 10 at 16:29
$begingroup$
$lceil 0.5rceil $=1
$endgroup$
– The Cat
Jan 10 at 16:29
1
1
$begingroup$
I think you just mistook those two together.
$endgroup$
– The Cat
Jan 10 at 16:30
$begingroup$
I think you just mistook those two together.
$endgroup$
– The Cat
Jan 10 at 16:30
1
1
$begingroup$
Yeah, now everything is good! Big thanks for you, and shoutout!
$endgroup$
– Kukoz
Jan 10 at 16:33
$begingroup$
Yeah, now everything is good! Big thanks for you, and shoutout!
$endgroup$
– Kukoz
Jan 10 at 16:33
|
show 4 more comments
$begingroup$
Let the sequence be $a_1,a_2,a_3,...a_n$. Then $|Omega | = {nchoose 3} = {n(n-1)(n-2)over 6}$
and each good 3-sequence is uniquely defined by it's middle term and the difference $d$. So if middle term is $a_m$ then the difference can be $d,2d,...,(m-1)d$.
So if we have $n=2k$, then we have $$1+2+...+(k-1)+(k-1)+...+2+1 = k(k-1)$$
good 3-sequences. So the probability is $$P = {6k(k-1)over 4k(2k-1)(k-1)} = {3over 2(n-1)}$$
If $n=2k+1$ then we have $$1+2+...+(k-1)+k+(k-1)+...+2+1 = k(k-1)+k=k^2$$
good 3-sequences. So the probability is $$P = {6k^2over 2(2k+1)k(2k-1)} = {3(n-1)over 2n(n-2)}$$
$endgroup$
add a comment |
$begingroup$
Let the sequence be $a_1,a_2,a_3,...a_n$. Then $|Omega | = {nchoose 3} = {n(n-1)(n-2)over 6}$
and each good 3-sequence is uniquely defined by it's middle term and the difference $d$. So if middle term is $a_m$ then the difference can be $d,2d,...,(m-1)d$.
So if we have $n=2k$, then we have $$1+2+...+(k-1)+(k-1)+...+2+1 = k(k-1)$$
good 3-sequences. So the probability is $$P = {6k(k-1)over 4k(2k-1)(k-1)} = {3over 2(n-1)}$$
If $n=2k+1$ then we have $$1+2+...+(k-1)+k+(k-1)+...+2+1 = k(k-1)+k=k^2$$
good 3-sequences. So the probability is $$P = {6k^2over 2(2k+1)k(2k-1)} = {3(n-1)over 2n(n-2)}$$
$endgroup$
add a comment |
$begingroup$
Let the sequence be $a_1,a_2,a_3,...a_n$. Then $|Omega | = {nchoose 3} = {n(n-1)(n-2)over 6}$
and each good 3-sequence is uniquely defined by it's middle term and the difference $d$. So if middle term is $a_m$ then the difference can be $d,2d,...,(m-1)d$.
So if we have $n=2k$, then we have $$1+2+...+(k-1)+(k-1)+...+2+1 = k(k-1)$$
good 3-sequences. So the probability is $$P = {6k(k-1)over 4k(2k-1)(k-1)} = {3over 2(n-1)}$$
If $n=2k+1$ then we have $$1+2+...+(k-1)+k+(k-1)+...+2+1 = k(k-1)+k=k^2$$
good 3-sequences. So the probability is $$P = {6k^2over 2(2k+1)k(2k-1)} = {3(n-1)over 2n(n-2)}$$
$endgroup$
Let the sequence be $a_1,a_2,a_3,...a_n$. Then $|Omega | = {nchoose 3} = {n(n-1)(n-2)over 6}$
and each good 3-sequence is uniquely defined by it's middle term and the difference $d$. So if middle term is $a_m$ then the difference can be $d,2d,...,(m-1)d$.
So if we have $n=2k$, then we have $$1+2+...+(k-1)+(k-1)+...+2+1 = k(k-1)$$
good 3-sequences. So the probability is $$P = {6k(k-1)over 4k(2k-1)(k-1)} = {3over 2(n-1)}$$
If $n=2k+1$ then we have $$1+2+...+(k-1)+k+(k-1)+...+2+1 = k(k-1)+k=k^2$$
good 3-sequences. So the probability is $$P = {6k^2over 2(2k+1)k(2k-1)} = {3(n-1)over 2n(n-2)}$$
answered Jan 10 at 17:36
greedoidgreedoid
42.9k1153105
42.9k1153105
add a comment |
add a comment |
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2
$begingroup$
Is the progression finite?
$endgroup$
– EuxhenH
Jan 7 at 18:05
$begingroup$
I totally forgot about it :/. Yes, the progression is finite.
$endgroup$
– Kukoz
Jan 7 at 18:15