92,795 reputation
581169
bio website von-eitzen.de/math/tntrep.xml
location Bonn, Germany
age 48
visits member for 1 year, 7 months
seen 13 hours ago

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


Apr
16
comment A question about limsup and limif
What about $a_n=\frac{(-1))^n}{n}$, $A=-0.01$, $B=0.01$? We have $A<\liminf a_n=0=\limsup a_n<B$ and yet $A<a_n<B$ does not hold for $n<100$??
Apr
15
answered Show that two points from four are at a distance $\leq \sqrt{3}$ in an equilateral triangle.
Apr
15
answered Implication or Bidirectional in “x is a Prime”
Apr
15
comment What would be an example such that $aH=bH$ but $Ha \neq Hb$?
@Amr A non-normal subgroup in an abelian group would be fine as well, if one could find such :)
Apr
15
answered Is this already an equation/law that has been found?
Apr
15
comment Cyclic and abelian group
Is here power = order or power = exponent?
Apr
15
comment Suppose that $\sum a_i$ converges and that $a_i\geq0$ for all 𝑖.
Where do you get stuck? Use the definition and show that the partial sums are a Cauchy sequence.
Apr
15
comment Proving a relation is a total order relation
Are you insisting on the "either - or" here? I.e. do you consider "$\le$" or do you consider "$<$" an order relation? Or do you complain because the proof in this form would also erroneously "show" that "divides" is a total order on $\{-2^n,-2^{n-1},\ldots,-4,-2-1,1,2,4,\ldots,2^n\}$?
Apr
15
comment Prove $M$ is a Maximal Ideal in $\Bbb Z\times \Bbb Z$
$M$ is the kernel of $\mathbb Z\times\mathbb Z\to\mathbb Z/p\mathbb Z$, $(x,y)\mapsto x+p\mathbb Z$
Apr
15
comment Conditions on $f:[a,b]\to \mathbb{R}$ to ensure a finite number of crossings
It i sof course sufficient to study the case that $f$ is differentiable on $(a,b)$. And one sifficient condition is of course that $(f')^{-1}(\{0\})$ is finite.
Apr
15
comment How can I learn Real Analysis in 10 days?
Well, it is also possible to learn playing the piano in 88 days, and it is sufficient to learn one key per day.
Apr
15
answered Prove every finite extension of subfields of $\mathbb{C}$ is simple
Apr
15
revised Is every subgroup of the product of two cyclic groups is again a product of two cyclic groups?
added 1 characters in body
Apr
15
comment Concerning squarefree numbers with 2 primes and squarefrees with 3 primes.
... or in fact $N\in\1,2,3,4,5\}$
Apr
15
answered Why exactly are NAND and NOR the only universal binary logic functions?
Apr
15
comment House edge when playing blackjack with perfect memory
A good strategy would seem to be: Bet virtually nothing until the deck is almost completely used up. But what you can make of the final round when only a handful of (known) cards are left in the game, may not be much. Also before that, the player can raise his winning probabilities, possibly into the positive, but the exact strategy at each moment depends on the precise distribution of remaining cards.
Apr
15
answered Draw a picture of a cube with edges a+b, and show it cut by planes that divide each edge into a segment of length a and a segment of length b.
Apr
15
revised Proving a limit Epsilon Delta definition
added 4 characters in body
Apr
15
awarded  algorithms
Apr
14
comment Why is the inverse of this function not a function?
What is property (1) by the way?