110,303 reputation
6101204
bio website von-eitzen.de/math/tntrep.xml
location Bonn, Germany
age 48
visits member for 2 years
seen 1 hour ago

I did study math and had a knack for it, but I am sooo out of that business now ...


Sep
12
answered Prove that the $\lim_{x \rightarrow 0}f(x)=b$ is equivalent to the $\lim_{x \rightarrow 0}f(x^3)=b$
Sep
12
answered A seemingly simple fact about construction of maps proven categorically, i.e. by universal properties
Sep
12
answered Does the series $\sum (1+n^2)^{-1/4}$ converge or diverge?
Sep
12
comment Does the intersection of sets have a categorical interpretation?
From the view ov category theory, inclusion is not special. Hence the relation among $A,B,A\cap B$ is "indistinguishable" from any other three sets of same cardinality, even if the other sets are in fact disjoint.
Sep
12
answered A Question on continuity of a piecewise function
Sep
11
revised “Partitioning” an uncountable set “equally”
added 805 characters in body
Sep
11
answered The set of all $p \in \mathbb{C}[x]$ that can be expressed using only one occurrence of $x$.
Sep
10
revised Checking whether it is integer or not.
added 4 characters in body
Sep
10
answered “Partitioning” an uncountable set “equally”
Sep
10
comment Variation of Fermat's Theorem
Start with uniqueness. Then count.
Sep
10
comment If G is a group such that all of its proper subgroups are abelian, then G itself must be abelian
Indeed, the smallest finite nonabelian group is an obvious counterexample (even if one doesn't know that $S_3$ is the smallest nonabelian group)
Sep
9
awarded  Enlightened
Sep
9
awarded  Good Answer
Sep
9
awarded  Nice Answer
Sep
9
comment Perform $x_1^3+x_2^3+\cdots+x_n^3$ by basic symetric polynomials.
Start with $\sigma_1^3$ and see what's too much ...
Sep
9
comment Proving the convergence of $a_{n+2}=(a_{n+1}a_n)^{1/2} \qquad (a_1\ge0, a_2\ge0)$
Show that the map $[0,\infty)^2\to[0,\infty)^2$, $(x,y)\mapsto (y,\sqrt {xy})$ is a contraction
Sep
9
revised Flipping a matrix?
added 170 characters in body
Sep
9
answered Flipping a matrix?
Sep
9
comment Prove by contraposition, if n is a positive integer such that n(mod3)=2 then n is not a perfect square
"if n is a positive integer such that n(mod3)=2 then n is a perfect square." is false. "Suppose if n is a negative integer such that n(mod3)=2 then n is not a perfect square." is true, but not the negation of the former
Sep
8
answered Show $\left|B^A\right| \cdot \left|B^A\right| = \left|B^A\right|$