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 Mar22 comment Determinant of a Vandermonde matrix of roots of monic polynomial with integer coefficients Ah, right, that's true... was too quick there, haha. Mar22 answered Determinant of a Vandermonde matrix of roots of monic polynomial with integer coefficients Dec8 awarded Caucus Sep5 answered Continuity of conjugate of $z$: $f(z)=\bar z$ Sep4 comment Polygons inscribed in circles, with integer sides and integer radius Note the similarity with the question about existence of perfect cuboids, i.e., cuboids with integer side lengths, integer side diameters, and integer space diameters. Sep4 asked Polygons inscribed in circles, with integer sides and integer radius Aug11 comment Tiling with polyominos You mean, Taylor discovered a tile that can only tile in a non-periodical manner. Aug8 awarded Yearling Jul2 awarded Curious Jun7 comment On combining $n$ and $n^2$ into one number @BenjaminDickman: Why discriminate on age? Perhaps it might be suitable for math.stackexchange, but it is seemingly a non-trivial question. Jun7 comment Show, by the element method that, for all subsets P, Q, and R of U, (P − Q) ∩ (R − Q) = (P ∩ R) − Q. Draw a picture. Venn diagrams is your friend. Clarification: This type of problem, can easily be proved using Venn diagrams. Even an elementary picture, can be considered as a proof. In this case, draw 3 circles, as on the wikipedia page, and identify left hand side, and right hand side in the picture. Note that the regions for both "interpretations" are the same. Jun7 answered Is $i$ irrational? Jun7 comment Number of solutions to sudoku puzzle @Jeff: General Sudoku ($n^2 \times n^2$ board) is NP-complete. The and the 9x9-board can be reduced to a SAT-problem, so it depends on your efficiency of the SAT-solver. However, this is equivalent to trying to solve the puzzle, I would say. If there was a quick way for 9x9-then most likely, this generalizes to all sizes, and you would become famous/assassinated by CIA. Jun7 asked Number of solutions to sudoku puzzle Jun1 comment Evaluate $\int_{0}^{\frac{\pi}{2}}\frac{1}{(1+x^2)(1+\tan x)}\,\mathrm dx$ I think this is a typo; tan should perhaps be arctan in the question. Then things makes sense. May29 comment compact and convex set How about using weak inequality in your definition, instead of strict? Is that what you mean? What are $X_1,X_2$ and $H$? Vectors? Mar22 awarded Tumbleweed Mar9 comment Invertibility of NxN nonnegative matrix with diagonally dominant elements As Benoit tells you; the intuition is as follows; for any such generic matrix, all elements are non-equal. Thus, we may fiddle each element a little bit and still satisfy all conditions you describe. Thus, there is an open ball around each generic matrix. If the generic matrix you started with was non-invertible, then some (most) elements in the small ball around this matrix must be invertible. Dec8 answered Circular Permutations With Repetitions (Mirrored Ignored) Nov30 comment Fields that require both CS and pure math Added now, see link.