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 Apr 4 comment Evaluate the sum with special function Numerically, it seems that the answer is 5 Dec 15 awarded Yearling Nov 13 awarded Notable Question Sep 8 comment vector division? @Salihcyilmaz True. I was answering the general question about vectors. Sep 8 answered vector division? Aug 29 answered Limiting value of $\frac{x^n e^x}{n!}$ as $n\to\infty$ Jun 7 comment Prove properties of $e^A$ when $A$ is a matrix The second statement, as it stands, is wrong. As a counterexample, take $A$ to be the zero matrix. May 20 accepted A maximization problem within the simplex May 14 comment Is it always true that the complement of a closed set is open? This reminds me of my lecturer in General Topology. He was quite an ambiguous fellow, and was rarely clear or precise about what he said. When introducing the concept of an open/closed set he told us "The door parable", which was: "A set is like a door. It can be either open, or closed. Except that a set can also be neither. Oh, and also both. You know what, forget about it..." May 5 comment A maximization problem within the simplex @copper.hat Yup. You're right. How didn't I think of that? You want to post an answer? May 5 asked A maximization problem within the simplex Apr 9 awarded Popular Question Mar 30 comment Linear regression of time series data - moving linear regression I don't know if this can be done in a simple way, but as a start I'd note that least-square fitting is done by calculating the inverse of a matrix of the form $A^TA$ where $A$ is a matrix constructed from the observations. Updating the inverse in each incremental step might be done via the Sherman-Morrison formula Mar 30 comment Linear regression of time series data - moving linear regression Why not use weights, that decrease (say, exponentially), the further you go into the past? That is, use a standard linear regression with weights $w_i=e^{-\lambda i}$ where $w_i$ is the weight of the $i$-th measurement, and the most recent measurement is $i=1$. $\lambda$ is a parameter that determines how much you want to bias your estimate towards more recent data. Mar 29 comment Minimal distance between points on two graphs Nice question. I don't have time to solve it now, but I think that defining the distance function as $$f(x,y)=(x-y)^2+(\sin(x)+2-\sin(y))^2$$ and moving to the variables $$x'=x-y,\qquad y'=x+y$$ might greatly simplify things. Mar 8 accepted When is the inverse of a sparse matrix dense? Feb 12 asked When is the inverse of a sparse matrix dense? Feb 1 revised There is relation that is symmetric and transitive but not reflexive? added 8 characters in body Feb 1 comment There is relation that is symmetric and transitive but not reflexive? OK. I still think it's a matter of taste, but I'll edit my answer. Feb 1 comment There is relation that is symmetric and transitive but not reflexive? That's a matter of taste. If you assume that $y$ exists and that $y=x$ then it's like assuming reflexivity in the first place, which is kind of cheating.