110,198 reputation
6101203
bio website von-eitzen.de/math/tntrep.xml
location Bonn, Germany
age 48
visits member for 2 years
seen 2 hours ago

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


4m
awarded  Nice Answer
2h
comment Is this set neccesarily to be a vector space?
If $S$ is a subset of a vector space $V$, then the given conditions are sufficient to make $S$ a subspace of $V$. But without such $V$, the addition and multiplication are not related enough
2h
comment Proof using induction on sequences
If $0\le x_{n-2}\le 1$ and $0\le x_{n-1}\le 1$, then the sine and cosine are both between $0$ and $1$. Of course you need both $n-1$ and $n-2$ for $n$, but that is still pretty weak (string would use all $k<n$). More precisely, strong induction for a statement $P(n)$ is weak induction for $\forall k<n\colon P(k)$ and here we do weak induction for $P(n)\land P(n-1)$, if you prefer.
8h
awarded  set-theory
14h
answered What formula will tell if three vertices in 3d space are ordered clockwise or counter-clockwise from the point of view of a camera?
1d
comment Show that R is an equivalence relation on X for x, y in X iff f(x) = f(y)
What have you tried? This is the mother of all equivalence relations ...
1d
answered Two definitions of connectedness: are they equivalent?
Sep
12
comment Closest fixed point to a convex set
Still waaay too broad
Sep
12
comment Does the series $\sum (1+n^2)^{-1/4}$ converge or diverge?
@UdiBehar For the edited part I cannot say more than user172541 in his answer.
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)