What is the probability that each element in this string is non-zero?
Question: You are given two bitstrings a1, a2, .. a77 and b1, b2, .. b77 of length 77 In both bitstrings, each bit is 0 with probability 3/4, and 1 with probability 1/4 (independent of all other bits).
Consider the string: a1-b1, a2-b2, ..., a77-b77.
What is the probability that each element in this string is non-zero?
Answer: 1.586381421*10^-33
Attempt:
Non-zero probability is 1/4 for both bitstrings.
If both bitstrings are subtracted by one another for length 77, then it shouldn't it be (1/4)*(1/4) = 1/16 be the probability of the new string being non-zero
Total number of possible bitstrings = 2^77
Probability: 1/16/2^77 = 4.13*10^-25
probability probability-theory discrete-mathematics
add a comment |
Question: You are given two bitstrings a1, a2, .. a77 and b1, b2, .. b77 of length 77 In both bitstrings, each bit is 0 with probability 3/4, and 1 with probability 1/4 (independent of all other bits).
Consider the string: a1-b1, a2-b2, ..., a77-b77.
What is the probability that each element in this string is non-zero?
Answer: 1.586381421*10^-33
Attempt:
Non-zero probability is 1/4 for both bitstrings.
If both bitstrings are subtracted by one another for length 77, then it shouldn't it be (1/4)*(1/4) = 1/16 be the probability of the new string being non-zero
Total number of possible bitstrings = 2^77
Probability: 1/16/2^77 = 4.13*10^-25
probability probability-theory discrete-mathematics
$a_1-b_1neq 0$ just means that $a_1neq b_1$. It certainly does not mean that both of the elements are $1$. Also, while it is true that there are $2^{77}$ possible strings, they are not equi-probable so you can't use that as a denominator in the way you have.
– lulu
Nov 26 at 21:21
Here's a MathJax tutorial :)
– Shaun
Nov 26 at 21:21
As a suggestion: start with a shorter string. What is the probability if both strings have length $1$? Length $2$? Length $3$?
– lulu
Nov 26 at 21:23
@lulu Isn't the question basically asking to find the probability of each element being non-zero so meaning it has to be 1? Bitstrings have either 0 or 1, so according to this, non-zero bitstrings = bitstrings with only 1s?
– Toby
Nov 26 at 21:44
1
No. The difference is not itself a bit string. if $a_1=0,b_1=1$ then $a_1-b_1=-1$ which is non-zero.
– lulu
Nov 26 at 22:05
add a comment |
Question: You are given two bitstrings a1, a2, .. a77 and b1, b2, .. b77 of length 77 In both bitstrings, each bit is 0 with probability 3/4, and 1 with probability 1/4 (independent of all other bits).
Consider the string: a1-b1, a2-b2, ..., a77-b77.
What is the probability that each element in this string is non-zero?
Answer: 1.586381421*10^-33
Attempt:
Non-zero probability is 1/4 for both bitstrings.
If both bitstrings are subtracted by one another for length 77, then it shouldn't it be (1/4)*(1/4) = 1/16 be the probability of the new string being non-zero
Total number of possible bitstrings = 2^77
Probability: 1/16/2^77 = 4.13*10^-25
probability probability-theory discrete-mathematics
Question: You are given two bitstrings a1, a2, .. a77 and b1, b2, .. b77 of length 77 In both bitstrings, each bit is 0 with probability 3/4, and 1 with probability 1/4 (independent of all other bits).
Consider the string: a1-b1, a2-b2, ..., a77-b77.
What is the probability that each element in this string is non-zero?
Answer: 1.586381421*10^-33
Attempt:
Non-zero probability is 1/4 for both bitstrings.
If both bitstrings are subtracted by one another for length 77, then it shouldn't it be (1/4)*(1/4) = 1/16 be the probability of the new string being non-zero
Total number of possible bitstrings = 2^77
Probability: 1/16/2^77 = 4.13*10^-25
probability probability-theory discrete-mathematics
probability probability-theory discrete-mathematics
edited Nov 26 at 21:21
Shaun
8,627113680
8,627113680
asked Nov 26 at 21:18
Toby
1577
1577
$a_1-b_1neq 0$ just means that $a_1neq b_1$. It certainly does not mean that both of the elements are $1$. Also, while it is true that there are $2^{77}$ possible strings, they are not equi-probable so you can't use that as a denominator in the way you have.
– lulu
Nov 26 at 21:21
Here's a MathJax tutorial :)
– Shaun
Nov 26 at 21:21
As a suggestion: start with a shorter string. What is the probability if both strings have length $1$? Length $2$? Length $3$?
– lulu
Nov 26 at 21:23
@lulu Isn't the question basically asking to find the probability of each element being non-zero so meaning it has to be 1? Bitstrings have either 0 or 1, so according to this, non-zero bitstrings = bitstrings with only 1s?
– Toby
Nov 26 at 21:44
1
No. The difference is not itself a bit string. if $a_1=0,b_1=1$ then $a_1-b_1=-1$ which is non-zero.
– lulu
Nov 26 at 22:05
add a comment |
$a_1-b_1neq 0$ just means that $a_1neq b_1$. It certainly does not mean that both of the elements are $1$. Also, while it is true that there are $2^{77}$ possible strings, they are not equi-probable so you can't use that as a denominator in the way you have.
– lulu
Nov 26 at 21:21
Here's a MathJax tutorial :)
– Shaun
Nov 26 at 21:21
As a suggestion: start with a shorter string. What is the probability if both strings have length $1$? Length $2$? Length $3$?
– lulu
Nov 26 at 21:23
@lulu Isn't the question basically asking to find the probability of each element being non-zero so meaning it has to be 1? Bitstrings have either 0 or 1, so according to this, non-zero bitstrings = bitstrings with only 1s?
– Toby
Nov 26 at 21:44
1
No. The difference is not itself a bit string. if $a_1=0,b_1=1$ then $a_1-b_1=-1$ which is non-zero.
– lulu
Nov 26 at 22:05
$a_1-b_1neq 0$ just means that $a_1neq b_1$. It certainly does not mean that both of the elements are $1$. Also, while it is true that there are $2^{77}$ possible strings, they are not equi-probable so you can't use that as a denominator in the way you have.
– lulu
Nov 26 at 21:21
$a_1-b_1neq 0$ just means that $a_1neq b_1$. It certainly does not mean that both of the elements are $1$. Also, while it is true that there are $2^{77}$ possible strings, they are not equi-probable so you can't use that as a denominator in the way you have.
– lulu
Nov 26 at 21:21
Here's a MathJax tutorial :)
– Shaun
Nov 26 at 21:21
Here's a MathJax tutorial :)
– Shaun
Nov 26 at 21:21
As a suggestion: start with a shorter string. What is the probability if both strings have length $1$? Length $2$? Length $3$?
– lulu
Nov 26 at 21:23
As a suggestion: start with a shorter string. What is the probability if both strings have length $1$? Length $2$? Length $3$?
– lulu
Nov 26 at 21:23
@lulu Isn't the question basically asking to find the probability of each element being non-zero so meaning it has to be 1? Bitstrings have either 0 or 1, so according to this, non-zero bitstrings = bitstrings with only 1s?
– Toby
Nov 26 at 21:44
@lulu Isn't the question basically asking to find the probability of each element being non-zero so meaning it has to be 1? Bitstrings have either 0 or 1, so according to this, non-zero bitstrings = bitstrings with only 1s?
– Toby
Nov 26 at 21:44
1
1
No. The difference is not itself a bit string. if $a_1=0,b_1=1$ then $a_1-b_1=-1$ which is non-zero.
– lulu
Nov 26 at 22:05
No. The difference is not itself a bit string. if $a_1=0,b_1=1$ then $a_1-b_1=-1$ which is non-zero.
– lulu
Nov 26 at 22:05
add a comment |
3 Answers
3
active
oldest
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Let $c_i=a_i-b_i$. You need to find $$P[c_1 neq 0, c_2 neq 0, ldots, c_{77} neq 0] = p^{77}$$
where $$p=P[c_i neq 0] = P[a_i = 0, b_i = 1] + P[a_i = 1, b_i = 0] = 2timesfrac{3}{16}=frac{6}{16}=frac{3}{8}$$
So, the final answer is $$left(frac{3}{8}right)^{77}$$ which gives the answer you provided.
Why is p to the power 77? Also, why does ai and bi have to be pairs of (0,1) and (1,0)? Is it to satisfy the ai - bi != 0 equation?
– Toby
Nov 26 at 21:54
1
It's $p^{77}$ because the individual elements ${c_i}$ are independent (because ${a_i} text{ and }{b_i}$ are independent). Yes, $c_ineq 0$ only if $a_ineq b_i$, so, you need to consider the cases where this holds. Note that $c_ineq 0$ means that $c_i = 1$ or $c_i = -1$.
– BlackMath
Nov 26 at 21:58
add a comment |
$a_i-b_ineq0implies a_ineq b_i$
The possible pairs $(a_i, b_i)$ are $(0, 1)$ and $(1,0)$, with each pair having a probability $frac{3}{4}cdotfrac{1}{4}=frac{3}{16}$. Therefore, the total probability is $(frac{3}{16}+frac{3}{16})^{77}=(frac{6}{16})^{77}$.
why does the probability have to be to the power 77?
– Toby
Nov 26 at 21:56
1
Because you have $77$ pairs of corresponding bits $(a_i, b_i)$, and the probability that the difference $a_i-b_ineq0$ is $6/16$ for all pairs.
– Shubham Johri
Nov 26 at 22:02
add a comment |
The answer is $(frac{6}{16})^{77}$.
One digit i being 0 is equivalent to $lnot(a_i=b_i=0lor a_i=b_i=1)$
can you please elaborate on how you got the answer. I don't follow your logic
– Toby
Nov 26 at 21:39
add a comment |
Your Answer
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3 Answers
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3 Answers
3
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Let $c_i=a_i-b_i$. You need to find $$P[c_1 neq 0, c_2 neq 0, ldots, c_{77} neq 0] = p^{77}$$
where $$p=P[c_i neq 0] = P[a_i = 0, b_i = 1] + P[a_i = 1, b_i = 0] = 2timesfrac{3}{16}=frac{6}{16}=frac{3}{8}$$
So, the final answer is $$left(frac{3}{8}right)^{77}$$ which gives the answer you provided.
Why is p to the power 77? Also, why does ai and bi have to be pairs of (0,1) and (1,0)? Is it to satisfy the ai - bi != 0 equation?
– Toby
Nov 26 at 21:54
1
It's $p^{77}$ because the individual elements ${c_i}$ are independent (because ${a_i} text{ and }{b_i}$ are independent). Yes, $c_ineq 0$ only if $a_ineq b_i$, so, you need to consider the cases where this holds. Note that $c_ineq 0$ means that $c_i = 1$ or $c_i = -1$.
– BlackMath
Nov 26 at 21:58
add a comment |
Let $c_i=a_i-b_i$. You need to find $$P[c_1 neq 0, c_2 neq 0, ldots, c_{77} neq 0] = p^{77}$$
where $$p=P[c_i neq 0] = P[a_i = 0, b_i = 1] + P[a_i = 1, b_i = 0] = 2timesfrac{3}{16}=frac{6}{16}=frac{3}{8}$$
So, the final answer is $$left(frac{3}{8}right)^{77}$$ which gives the answer you provided.
Why is p to the power 77? Also, why does ai and bi have to be pairs of (0,1) and (1,0)? Is it to satisfy the ai - bi != 0 equation?
– Toby
Nov 26 at 21:54
1
It's $p^{77}$ because the individual elements ${c_i}$ are independent (because ${a_i} text{ and }{b_i}$ are independent). Yes, $c_ineq 0$ only if $a_ineq b_i$, so, you need to consider the cases where this holds. Note that $c_ineq 0$ means that $c_i = 1$ or $c_i = -1$.
– BlackMath
Nov 26 at 21:58
add a comment |
Let $c_i=a_i-b_i$. You need to find $$P[c_1 neq 0, c_2 neq 0, ldots, c_{77} neq 0] = p^{77}$$
where $$p=P[c_i neq 0] = P[a_i = 0, b_i = 1] + P[a_i = 1, b_i = 0] = 2timesfrac{3}{16}=frac{6}{16}=frac{3}{8}$$
So, the final answer is $$left(frac{3}{8}right)^{77}$$ which gives the answer you provided.
Let $c_i=a_i-b_i$. You need to find $$P[c_1 neq 0, c_2 neq 0, ldots, c_{77} neq 0] = p^{77}$$
where $$p=P[c_i neq 0] = P[a_i = 0, b_i = 1] + P[a_i = 1, b_i = 0] = 2timesfrac{3}{16}=frac{6}{16}=frac{3}{8}$$
So, the final answer is $$left(frac{3}{8}right)^{77}$$ which gives the answer you provided.
answered Nov 26 at 21:48
BlackMath
18018
18018
Why is p to the power 77? Also, why does ai and bi have to be pairs of (0,1) and (1,0)? Is it to satisfy the ai - bi != 0 equation?
– Toby
Nov 26 at 21:54
1
It's $p^{77}$ because the individual elements ${c_i}$ are independent (because ${a_i} text{ and }{b_i}$ are independent). Yes, $c_ineq 0$ only if $a_ineq b_i$, so, you need to consider the cases where this holds. Note that $c_ineq 0$ means that $c_i = 1$ or $c_i = -1$.
– BlackMath
Nov 26 at 21:58
add a comment |
Why is p to the power 77? Also, why does ai and bi have to be pairs of (0,1) and (1,0)? Is it to satisfy the ai - bi != 0 equation?
– Toby
Nov 26 at 21:54
1
It's $p^{77}$ because the individual elements ${c_i}$ are independent (because ${a_i} text{ and }{b_i}$ are independent). Yes, $c_ineq 0$ only if $a_ineq b_i$, so, you need to consider the cases where this holds. Note that $c_ineq 0$ means that $c_i = 1$ or $c_i = -1$.
– BlackMath
Nov 26 at 21:58
Why is p to the power 77? Also, why does ai and bi have to be pairs of (0,1) and (1,0)? Is it to satisfy the ai - bi != 0 equation?
– Toby
Nov 26 at 21:54
Why is p to the power 77? Also, why does ai and bi have to be pairs of (0,1) and (1,0)? Is it to satisfy the ai - bi != 0 equation?
– Toby
Nov 26 at 21:54
1
1
It's $p^{77}$ because the individual elements ${c_i}$ are independent (because ${a_i} text{ and }{b_i}$ are independent). Yes, $c_ineq 0$ only if $a_ineq b_i$, so, you need to consider the cases where this holds. Note that $c_ineq 0$ means that $c_i = 1$ or $c_i = -1$.
– BlackMath
Nov 26 at 21:58
It's $p^{77}$ because the individual elements ${c_i}$ are independent (because ${a_i} text{ and }{b_i}$ are independent). Yes, $c_ineq 0$ only if $a_ineq b_i$, so, you need to consider the cases where this holds. Note that $c_ineq 0$ means that $c_i = 1$ or $c_i = -1$.
– BlackMath
Nov 26 at 21:58
add a comment |
$a_i-b_ineq0implies a_ineq b_i$
The possible pairs $(a_i, b_i)$ are $(0, 1)$ and $(1,0)$, with each pair having a probability $frac{3}{4}cdotfrac{1}{4}=frac{3}{16}$. Therefore, the total probability is $(frac{3}{16}+frac{3}{16})^{77}=(frac{6}{16})^{77}$.
why does the probability have to be to the power 77?
– Toby
Nov 26 at 21:56
1
Because you have $77$ pairs of corresponding bits $(a_i, b_i)$, and the probability that the difference $a_i-b_ineq0$ is $6/16$ for all pairs.
– Shubham Johri
Nov 26 at 22:02
add a comment |
$a_i-b_ineq0implies a_ineq b_i$
The possible pairs $(a_i, b_i)$ are $(0, 1)$ and $(1,0)$, with each pair having a probability $frac{3}{4}cdotfrac{1}{4}=frac{3}{16}$. Therefore, the total probability is $(frac{3}{16}+frac{3}{16})^{77}=(frac{6}{16})^{77}$.
why does the probability have to be to the power 77?
– Toby
Nov 26 at 21:56
1
Because you have $77$ pairs of corresponding bits $(a_i, b_i)$, and the probability that the difference $a_i-b_ineq0$ is $6/16$ for all pairs.
– Shubham Johri
Nov 26 at 22:02
add a comment |
$a_i-b_ineq0implies a_ineq b_i$
The possible pairs $(a_i, b_i)$ are $(0, 1)$ and $(1,0)$, with each pair having a probability $frac{3}{4}cdotfrac{1}{4}=frac{3}{16}$. Therefore, the total probability is $(frac{3}{16}+frac{3}{16})^{77}=(frac{6}{16})^{77}$.
$a_i-b_ineq0implies a_ineq b_i$
The possible pairs $(a_i, b_i)$ are $(0, 1)$ and $(1,0)$, with each pair having a probability $frac{3}{4}cdotfrac{1}{4}=frac{3}{16}$. Therefore, the total probability is $(frac{3}{16}+frac{3}{16})^{77}=(frac{6}{16})^{77}$.
answered Nov 26 at 21:47
Shubham Johri
3,826716
3,826716
why does the probability have to be to the power 77?
– Toby
Nov 26 at 21:56
1
Because you have $77$ pairs of corresponding bits $(a_i, b_i)$, and the probability that the difference $a_i-b_ineq0$ is $6/16$ for all pairs.
– Shubham Johri
Nov 26 at 22:02
add a comment |
why does the probability have to be to the power 77?
– Toby
Nov 26 at 21:56
1
Because you have $77$ pairs of corresponding bits $(a_i, b_i)$, and the probability that the difference $a_i-b_ineq0$ is $6/16$ for all pairs.
– Shubham Johri
Nov 26 at 22:02
why does the probability have to be to the power 77?
– Toby
Nov 26 at 21:56
why does the probability have to be to the power 77?
– Toby
Nov 26 at 21:56
1
1
Because you have $77$ pairs of corresponding bits $(a_i, b_i)$, and the probability that the difference $a_i-b_ineq0$ is $6/16$ for all pairs.
– Shubham Johri
Nov 26 at 22:02
Because you have $77$ pairs of corresponding bits $(a_i, b_i)$, and the probability that the difference $a_i-b_ineq0$ is $6/16$ for all pairs.
– Shubham Johri
Nov 26 at 22:02
add a comment |
The answer is $(frac{6}{16})^{77}$.
One digit i being 0 is equivalent to $lnot(a_i=b_i=0lor a_i=b_i=1)$
can you please elaborate on how you got the answer. I don't follow your logic
– Toby
Nov 26 at 21:39
add a comment |
The answer is $(frac{6}{16})^{77}$.
One digit i being 0 is equivalent to $lnot(a_i=b_i=0lor a_i=b_i=1)$
can you please elaborate on how you got the answer. I don't follow your logic
– Toby
Nov 26 at 21:39
add a comment |
The answer is $(frac{6}{16})^{77}$.
One digit i being 0 is equivalent to $lnot(a_i=b_i=0lor a_i=b_i=1)$
The answer is $(frac{6}{16})^{77}$.
One digit i being 0 is equivalent to $lnot(a_i=b_i=0lor a_i=b_i=1)$
answered Nov 26 at 21:28
Martin Erhardt
1859
1859
can you please elaborate on how you got the answer. I don't follow your logic
– Toby
Nov 26 at 21:39
add a comment |
can you please elaborate on how you got the answer. I don't follow your logic
– Toby
Nov 26 at 21:39
can you please elaborate on how you got the answer. I don't follow your logic
– Toby
Nov 26 at 21:39
can you please elaborate on how you got the answer. I don't follow your logic
– Toby
Nov 26 at 21:39
add a comment |
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$a_1-b_1neq 0$ just means that $a_1neq b_1$. It certainly does not mean that both of the elements are $1$. Also, while it is true that there are $2^{77}$ possible strings, they are not equi-probable so you can't use that as a denominator in the way you have.
– lulu
Nov 26 at 21:21
Here's a MathJax tutorial :)
– Shaun
Nov 26 at 21:21
As a suggestion: start with a shorter string. What is the probability if both strings have length $1$? Length $2$? Length $3$?
– lulu
Nov 26 at 21:23
@lulu Isn't the question basically asking to find the probability of each element being non-zero so meaning it has to be 1? Bitstrings have either 0 or 1, so according to this, non-zero bitstrings = bitstrings with only 1s?
– Toby
Nov 26 at 21:44
1
No. The difference is not itself a bit string. if $a_1=0,b_1=1$ then $a_1-b_1=-1$ which is non-zero.
– lulu
Nov 26 at 22:05