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comment graph theory - clique graph
There is also an entry in Wikipedia.
comment Graph and one Sequence challenge
@MioMina I do not see (d), and I can confirm if some of them are correct, but I have to see your work first (in particular the derivation, not just the answer).
revised Graph and one Sequence challenge
added 50 characters in body
answered Graph and one Sequence challenge
answered Every nonhamiltonian 2-connected graph has a theta subgraph
comment Improving an algorithm.
You sort the files by size at first, so in the "standard" computational model it is $O(n \log n)$ already. Also, you can't do it faster than touching each file at least once which gives you linear lower bound on the number of operations. On the other hand, you can do it in the parallel, that is, e.g. you can put files of different sizes into different buckets and compare different buckets using multiple processors.
comment relationship between Minkowski addition and the trajectory of a numerical controle machine?
You might be confused, because that's not quite true. Let $A$ be the shape of the cutting tool, and $P$ the trajectory of the cut. The shape of the cut is the sum of the translations of $A$ along the curve $P$. Minkowski sum gives you something similar, but not exactly, for example if $A$ is a circle of radius $1\mathrm{unit}$ and $P$ consists of edges of a square of size $10\mathrm{units}$, then the cut would be a rounded-corner square of size $11\mathrm{units}$ with the thickness $1\mathrm{unit}$ while the Minkowski sum will get you thickness $2\mathrm{units}$ and size $12\mathrm{units}$.
revised Select one or zero elements from a set
added 536 characters in body
answered Select one or zero elements from a set
comment Graph and one Sequence challenge
See here (there's an inequality that generalizes to directed graphs), there's also a related question here (note that your graph is directed, where in that question is undirected).
comment Distinguish between left and right polygon
Assuming your polygons are simple ("land information data" indicates this might be the case), you can calculate their signed area and check its sign, e.g. see here.
comment question on morse code
Please note, that Morse code uses in fact more characters. Even if you disregard the pause between words (7x dot) and incorporate the between-elements pause (1x dot) into the elements (e.g. dot would be 10 and dash 1110), then there is still the pause between letters (3x dot, or 00).
asked Basic examples of probabilistic method
comment Determine sign of sum of square roots
It is possible, but the solution that comes now to my mind isn't trivial. If you would consider field extensions $\mathbb{Q}[\sqrt{r_i},\ldots]$, then it should be possible to keep everything in rational numbers and possibly even in integers. However, such extensions behave a lot like dimensions ($\mathbb{R}[\sqrt{-1}]$ is exactly $\mathbb{C}$, which is a lot like $\mathbb{R}^2$), so there might be some linear equations involved. There might be some easier solution, but right now that's the only one I have.
comment Will it become impossible to learn math?
@MikeMiller Did you both receive notifications for this comment?
answered Discrete Mathemetics - Hamilton Path
answered Proof between max independent set cardinal and min vertex cover.
comment On the average length of the Steiner net for $n$ randomly chosen points in the unit square
Don't asymptotics for Euclidean MSTs imply anything?
comment Geometric problem with a lvl between TST and IMO.
@Blue I knew there was a simpler solution (I was using the dual of the one you mention + Desargues').
comment Geometric problem with a lvl between TST and IMO.
Care to make a diagram?