Johannes Kloos
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 19h reviewed Approve $y_{2n}, y_{2n+1}$ and $y_{3n}$ all converge. What can we say about the sequence $y_n$? 19h reviewed Approve Proving $T(n) = 1 + \sum_{j=0}^{n-1} T(j)$, $T(0)=1$ implies $T(n)=2^n$ Apr21 reviewed Leave Open Common terms between two arithmetic series Apr21 reviewed Leave Open A proof about boundedness for continuous functions Apr21 reviewed Close Martingale difference sequence Apr21 reviewed Leave Open Why $R/AB$ is cyclic? Apr21 reviewed Approve Dropping the lowest number in a set lowers the mean average of the set. Apr21 reviewed Approve Riemann Sum proofs Apr12 awarded Nice Answer Mar31 comment Push Down Automata What have you tried so far? Also, do you know how to construct a PDA for $a^nb^n$? If you have one for $a^nb^n$ and one for $a^nc^n$, what can you do with that? Mar5 awarded Yearling Feb22 comment When is matrix multiplication commutative? Here's an example why this condition is not neccesary: Take any non-diagonalizable matrix $A$. It will always commute with the unit matrix $1$, but $A$ and $1$ are clearly not simultaneously diagonalizable. Feb12 comment When is matrix multiplication commutative? It's not neccesary, AFAICT. I can't think of a counter-example right know, though. Jan31 awarded Revival Jan22 comment $f(x) \in R[X]$ irreducible $\Rightarrow (f(x))$ ideal? Just to make sure: Do you mean by $(f)$ the ideal generated by $f$? Jan11 comment Powers of $2\times 2$ matrices, such that $A^n = I$ Do you know about rotation matrices? Dec19 awarded Caucus Sep30 awarded Explainer Sep25 awarded Nice Answer Sep13 comment What is a polynomially bounded function? Your argument would be correct if, say, $2^x-1$ was actually a polynomial. But by definition, a polynomial is of the form $a_n x^n + a^{n-1} x^{n-1} + \cdots + a_0$ for some parameters $a_0, \ldots, a_n$, and $2^x-1$ is not of this form.