18,363 reputation
638109
bio website tancast.com/wp-content/…
location
age
visits member for 3 years, 10 months
seen 22 hours ago

Oct
20
comment Show that the directional derivative is linear by definition
I'm sorry but could you help me understand this? What I understand so far: In higher dimension the definition of the derivative involves a matrix so it is linear. On the other hand, the directional derivative does not seem to include such a matrix. Why does it nonetheless follow from the definition of directional derivative that it is linear? Thanks a lot in advance.
Oct
19
revised Complete additivity of set functions
edited title
Oct
19
comment Partitions without 2
Not as active as before: Even with 50% activity you'd still be one of the more active users. Is less activity due to less interesting questions? Or is it my imagination that there are less interesting questions compared to 3 years ago?
Oct
16
comment Proof of that there is no metric on $\mathbb{R}$ which is equivalent to the natural metric and which induces a metric on $(0,1)$
Please can you provide your definition of when two metrics are equivalent?
Oct
16
revised Nullity and rank of a linear transformation
edited tags
Oct
15
revised Meaning of a discrete topological sub-space?
deleted 47 characters in body
Oct
15
comment Meaning of a discrete topological sub-space?
@Eric_ Yes, that's right. And sorry, I couldn't make sense of it before.
Oct
15
comment Meaning of a discrete topological sub-space?
Duh, beat me by 52 seconds and I didn't even get a notification that someone else posted an answer.
Oct
15
answered Meaning of a discrete topological sub-space?
Oct
14
comment Density of sets
@learningmaths I see you got an answer along the same vein from Yiorgos so I'll assume your question has been answered to your satisfaction. There's nothing I can add : )
Oct
14
comment Density of sets
I have to leave my keyboard right now and I might add an answer to the second question when I return.
Oct
14
comment Density of sets
+1 for providing a question I like thinking about.
Oct
14
answered Density of sets
Oct
11
awarded  Popular Question
Oct
11
comment Metric topology induced by the sum of two metrics
Yay! You're back. Cool.
Oct
10
awarded  Socratic
Oct
9
comment Follow up on a previous question of mine (characters in star algebra)
@MartinArgerami True but I still don't understand what's going on here. Concretely, what is OP doing wrong in the proof in the question? In the question the C star algebra is not zero so you can pick any non-zero element $a$ and use that $r(a^\ast a) = \|a^\ast a\|>0$, no?
Oct
9
comment Follow up on a previous question of mine (characters in star algebra)
Also every proper modular ideal is contained in a maximal ideal so even if there is no $1$ it seems plausible that maximal ideals exist. (?)
Oct
9
comment Follow up on a previous question of mine (characters in star algebra)
Please could you enlighten me as to what's going on here? It's a theorem in Murphy's book (see thm. 1.3.4. on page 14) that in a non-unital abelian Banach algebra $A$ we have $\sigma (a) = \{\tau (a) \mid \tau \in \Omega (A)\} \cup \{0\}$, so surely there cannot be a counterexample. Thank you in advance.
Oct
5
comment Every homomorphism on a C*-algebra is a *-homomorphism
Okay, but I didn't understand why it was not correct...(Sorry, I only saw your reply now)