15,657 reputation
12042
bio website math.brown.edu/~lubinj
location Minnesota
age 78
visits member for 3 years, 3 months
seen 8 hours ago

Aged mathematician


Jan
13
comment How to pronounce “Sturm”?
Seems to be the inverse problem to how to pronounce the name of the German mathematician Lejeune Dirichlet.
Jan
12
answered Explicitly computing uniformisers of local fields
Jan
12
answered What is $0^{i}$?
Jan
12
comment Localization of a ring that is not an integral domain
I think you have to look closely at the correct definition of localization. For rings with zero-divisors, there may be some pitfalls.
Jan
12
comment What is $0^{i}$?
I think that OP meant $i$ to be the imaginary unit.
Jan
12
comment What are the ring homomorphisms $\mathbb{Z}^\mathbb{N} \to \mathbb{Z}$?
@Ivan, his ring is not $\oplus\Bbb Z$.
Jan
11
comment What are the ring homomorphisms $\mathbb{Z}^\mathbb{N} \to \mathbb{Z}$?
I think that we need more information. Are both multiplication and addition componentwise? You allow infinitely many nonzero entries, I guess, so that the multiplicative unit would be $1$ everywhere?
Jan
11
revised How many elements does $\mathbb Z_7[i]/\langle i+1\rangle$ have?
added 28 characters in body
Jan
11
answered How many elements does $\mathbb Z_7[i]/\langle i+1\rangle$ have?
Jan
11
comment How many elements does $\mathbb Z_7[i]/\langle i+1\rangle$ have?
In total sympathy and agreement with @JyrkiLahtonen, I still don’t know what $\langle1+i\rangle$ is meant to be. Are we talking additive structure here or multiplicative, or, most unlikely, ideal structure?
Jan
10
answered Attempt on proving elements of $\Bbb Z[\sqrt{-3}]$ are relatively prime
Jan
10
comment Attempt on proving elements of $\Bbb Z[\sqrt{-3}]$ are relatively prime
Are you aware that $\Bbb Z[\sqrt{-3}\,]$ is not a principal ideal domain?
Jan
10
comment Prove that factor rings are fields
Not knowing the general theory, you could do this by working in the first field, and finding in it a root of $X^3-X+1$.
Jan
10
comment Prove that factor rings are fields
Right, but OP pretty clearly doesn’t have the requisite background yet for understanding this.
Jan
10
answered Equation of a circle using rational fractions
Jan
9
comment Standard matrix of rotation of unit cube
Seems to me that once you draw your mental picture, you see that these are just the three cyclic permutations of the coordinates. So your matrices are the three permutation matrices of determinant $+1$.
Jan
8
comment Construction of an ellipse
No, @Dasherman, it’s more like a parametrization of the ellipse $t\mapsto (a\cos t,b\sin t)$. It’s in Wikipedia as “Trammel of Archimedes“.
Jan
8
comment Construction of an ellipse
There also is (or used to be) an mechanical-drawing gadget called a trammel, it has also appeared as a children’s toy. Draws a very satisfying and mathematically correct ellipse. Accuracy depends on the tightness of the mechanical construction.
Jan
8
comment Find all the fields between $\mathbb{Q}$ and the splitting field of $x^4 + 81$
@HagenvonEitzen, I believe a typo has caused you to omit a comma: the splitting field is $\Bbb Q[i,\sqrt2\,]$.
Jan
8
comment Let $p$ be a prime , then $\{0\} \cup\bigl\{\frac ab \in \mathbb Q : a \ne 0 \space ; (a,b)=1=(p,b) \bigr\}$ is a subring of $(\mathbb Q,+,.)$?
@GPerez, right you are. That’s why I explained what I meant by the notation $\Bbb Z_p$.