The summations tag has no wiki summary.
1
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0answers
21 views
Sum of squared/cube combinations [migrated]
I was wondering if there is a closed formula for sum of cubed combinations. More precisely, I'd like to compute $$\sum_{k=1}^n \left ( \begin{array}{c}n\\k\end{array}\right )^3$$
Obviously, without ...
4
votes
1answer
117 views
What can we conclude about the distribution of the sum of two random variables?
If we know, for independent random variables $X$ and $Y$,
$P(X>x)\leq0.05$, and $P(Y>y)\leq0.05$, can we say anything about $P(X+Y>x+y)$? Can we be certain that it is less than $0.05$? Under ...
0
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1answer
139 views
How to rewrite a sum of probabilities formula as multiplications?
I have an equation like that:
$p(r|s)= \frac{p(s,r)}{p(s)}=\frac{ \sum_{w,c} p(c,s,r,w)}{\sum_{w,c,r} p(c,s,r,w)} $
I am new to probability and I want to learn that how can I write sum formula as ...
0
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1answer
272 views
Sum(XY) in terms of Xbar and Ybar [closed]
If $x$ and $y$ are two series, is there any relation between $\sum{(x,y)}$ that can be expressed in terms of mean of these two. Specifically, I want to know if any sort of relation exists between ...
2
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1answer
69 views
Getting past independence assumptions in modeling the sum of random variables (application in education)
I'm trying to model a student's semester GPA $G$ as a random variable.
Semester GPA is a weighted (based on credit hours) sum of a student's grade points (e.g. 0=F,1=D,2=C,3=B,4=A).
We can consider ...
0
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0answers
56 views
Calculate a discount rate given a PV at some point in the future [closed]
I am trying to calculate a 24-month Customer Lifetime Value for a hypothetical magazine subscription service. CLV is typically calculated as the summation of the present value of future cash fows from ...
2
votes
1answer
227 views
Deriving OLS estimates using method of moments
I've worked the slope all the way down to $\sum [x_i(y_i - \bar{y})] = \hat\beta_1 \sum[x_i(x_i - \bar{x})]$
But I can not figure out how to show the steps for:
$\sum[x_i(y_i - \bar{y})] = \sum(x_i ...
1
vote
1answer
68 views
Estimate variance of sub-sets from overall variance
I am looking for a way to estimate the variance of a summed sub-set based on the variance of those sums.
Si = sum( Ai )
S = { S0...Sn }
V = variance( S )
That ...
0
votes
0answers
81 views
Understanding summations in COV formula for time series
I am looking through Time Series Analysis: With Applications in R (my first exposure to time series) and refreshing summations.
I.
When given the following rule:
COV[$\sum_{i=1}^{m} ...
