128,138 reputation
9121250
bio website von-eitzen.de/math/tntrep.xml
location Bonn, Germany
age 49
visits member for 2 years, 5 months
seen yesterday

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


Jan
16
comment Are there infinitely many pairs of primes where one divides one more than the square of the other?
All your examples are of the form $F_{n-1}, F_{n+1}$ with $F_n$ denoting the $n$th Fibonacci numbre
Jan
16
answered How to aproach that this equation has infinite positive solutions?
Jan
16
comment Primes as sum of squares.
Your strategy is doomed to fail. After all, $p_j$ can always be written as sum of four squares, so this can hardly lead to a contradiction.
Jan
16
answered Is $\cos(1)^2$ irrational?
Jan
16
answered Does this graph contain $K_5$ or $K_{3,3}$ as subdivision or minor?
Jan
16
comment Why probability of intersection of independent events cannot be attained by multiplying
Given that 24/100 is not 20/100, how dare you assume that the events are independent?
Jan
16
comment Limits of floor functions
What are your thought? What are the values of $f(0.0001)$, $f(-0.0001)$, $f(0)$, $f(-0)$?
Jan
16
comment Proof that $f:\mathbb R\to[-1,1], f(x)=\cos x$ is surjective
Do you know that the cosine function is continuous? And which definition of cosine do you work with (geometric, power series, from complex exponential, ...)?
Jan
16
answered Prove that if $f$ is convex and upper bounded, it must be constant.
Jan
16
answered How to calculate what percentage a fraction is
Jan
15
answered can a number of the form $x^2 + 1 $ be a square number?
Jan
15
comment Algorithm for finding “fact families”
From the text I learn: A "multiplication fact" is one member of a "fact family" (and there may be several multiplication facts in a fact family) and $2\times 3=6$ is a fact family. Also, fact families have several "dividends". - This is totally weird. My best bet would be that a fact family is a collection of related facts, such as $a\cdot b=c$, $b\cdot a=c$, $c:a=b$, and $c:b=a$. That interpretation matches some of the information readable from the problem statement, but it clearly contradicts others.
Jan
15
comment How to prove that $f=id_A$?
@bolzanoman There are so few steps that it is hard to see what is meant with final step.- Given $x\in A$, we want to show that $f(x)=x$. By surjectivity of $f$, there exists $y$ with $x=f(y)$. Then by this and the property of $f$, we have $f(x)=f(f(y))=(f\circ f)(y)=f(y)=x$ as desired.
Jan
15
answered Sum involving the “distance to the nearest integer function”
Jan
15
comment Showing that the mapping $a\in M, a^3=1,x\mapsto ax^2a^2$ is an automorphism of $M$
As stated, the problem statement is wrong. It has already been noted that $M$ needs to be commutative. Moreover, $f$ may fail to be onto: Let $(M,\cdot)=(\mathbb N_0,+)$ and $a=0$. Then $f(x)$ is even for all $x$.
Jan
15
answered Nullary Arithmetic Product (at Wiki)
Jan
15
comment $e^{\pi\sqrt N}$ is very close to an integer for some smallish $N$s. What about $\pi^{e\sqrt N}$?
Instead of the last sentence I'd remark that $\pi^x$ is extremely dull and unnatural compared to $e^x$.
Jan
15
comment Open neighborhoods in the definition of a manifold
By what you say in the last paragraph, one might replace "neighbourhood" with "open neighbourhood" in the definition and still have the same theory
Jan
15
answered Continuous homomorphisms and differentiable homomorphisms on $\mathbb{R}$
Jan
15
answered Why can we write a uncontinual function continual?