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1d
comment If $I$ is a closed ideal in a C*-algebra $A$ and $J$ is a closed ideal in $I$ then $J$ is an ideal of $A$
How does $ab \in J$ follow from $ab^{1/2}\in I$? We have $b \in J^+$ but as far as I can tell there is no reason why we should also have $b^{1/2}$.
1d
comment A concave positive function on $[1,\infty)$ is uniformly continuous
@user161825 It's a pity that there is no badge for it.
1d
comment If $I$ is a closed ideal in a C*-algebra $A$ and $J$ is a closed ideal in $I$ then $J$ is an ideal of $A$
I don't understand your proof. Yes, for all $b \in J^+$ we also have $b \in I$ and therefore $ab,ba \in I$. But it doesn't follow that $ab^{1/2}$ or $ab$ are in $J$.
1d
revised Prove: $\frac1x$ is not a uniformly continuous function
deleted 4 characters in body
1d
comment Prove: $\frac1x$ is not a uniformly continuous function
@Bungo Thank you!
1d
answered Prove: $\frac1x$ is not a uniformly continuous function
Dec
9
awarded  Popular Question
Dec
9
revised Minkowski type inequality in Banach algebras
It's likely to be a Cauchy-Schwarz type inequality since Cauchy died in 1857 and Schwartz was born in 1915.
Dec
9
comment Minkowski type inequality in Banach algebras
Thanks. Now what about changing Cauchy-Schwartz to Cauchy-Schwarz?
Dec
9
comment Minkowski type inequality in Banach algebras
Yeah, just figured it out this very second. Duh. 6 minutes too late.
Dec
9
comment Minkowski type inequality in Banach algebras
Thank you for your comment, I understand. I have one question though: How do you get $AB^\ast = B^\ast A$ and $A^\ast B = B A^\ast$ from $A,B$ normal and commuting?
Dec
7
awarded  Popular Question
Dec
7
comment Minkowski type inequality in Banach algebras
Did you mean for $A+B$ normal? I assume you want to apply the Gelfand representation. In any case, I was wondering if I may change Cauchy-Schwartz to Cauchy-Schwarz.
Dec
6
comment Cardinality of set difference
@Victor Ok, can you post the solution in an answer of your own then? By now you probably know the answer and I'd be interested to see it too.
Dec
6
revised homeomorphism between $2^{\mathbb{N}}$ and the Cantor Middle third set
Unsolicited tag nuked.
Dec
6
answered Reals constructed from equivalence classes of Cauchy sequences of rationals.
Nov
30
answered Cardinality of set difference
Nov
28
comment Cancellation of Direct Product in Grp
How do you read $\times$-compact? "product compact"? Did you invent this definition?
Nov
25
comment $\Bbb{R}^2$ not homeomorphic to $\Bbb{R}^2\setminus \{0\}$
Your question was answered here: math.stackexchange.com/a/30888/5798
Nov
24
comment Circle to circle homotopic to the constant map?
Ah, great, I understand. I think I can write the proof. Thank you very much for your comment!