Robert Cardona's user avatar
Robert Cardona's user avatar
Robert Cardona's user avatar
Robert Cardona
  • Member for 12 years
  • Last seen more than 2 years ago
  • Barcelona, Spain
46 votes

A Banach space is reflexive if and only if its dual is reflexive

20 votes

Proof that $\mathbb Z[\sqrt{3}]$ is a Euclidean Domain

19 votes

If a connected open set is evenly covered, then its preimage is uniquely partitioned into slices

19 votes

The fiber of a covering space over a connected space has constant cardinality

19 votes

Proving that the set of limit points of a set is closed

19 votes

Show that $d_b(x,y)=\frac{d(x,y)}{1+d(x,y)}$ is a metric.

18 votes

Real numbers equipped with the metric $ d (x,y) = | \arctan(x) - \arctan(y)| $ is an incomplete metric space

16 votes
Accepted

$\mathbb Z[\sqrt{-5}]$ is not a UFD

13 votes
Accepted

Prove the increasing union of ideals is an ideal

12 votes
Accepted

$H$ normal in $G$. Need $G$ contain a subgroup isomorphic to $G/H$

12 votes
Accepted

Is the closure of any linear subspace of a normed space $X$ again a linear subspace of $X$?

12 votes

Every ideal contains a power of it's radical in a Noetherian ring.

11 votes
Accepted

Proving the quotient of a principal ideal domain by a prime ideal is again a principal ideal domain

11 votes

why is $\ell_0$ a pseudo-norm?

10 votes

How to prove that $L^p [0,1]$ isn't induced by an inner product? for $p\neq 2$

9 votes

Showing a function of two variables is measurable

9 votes

Nullhomotopic map extended

8 votes

$\langle 2,x \rangle$ is a non-principal ideal in $\mathbb Z [x];\, $ $D[x]$ PID $\iff D$ field, for a domain $D$

8 votes

Characterization of ideals in rings of fractions

8 votes

prove $\liminf\limits_{n\to\infty} A_{n} \subset \limsup \limits_{n\to\infty} A_{n}$

7 votes
Accepted

Is every positive sequence that tends to zero monotone decreasing?

7 votes

Prime ideals in $A$ and prime ideals in $S^{-1}A$

7 votes

Circle to circle homotopic to the constant map?

7 votes
Accepted

Number of homomorphism from $S_3$ to $\mathbb Z_6$

7 votes

Sum of Measures of Two Subsets = Sum of Measures of Their Union & Intersection

7 votes
Accepted

Suppose $R$ is a commutative ring with unity such that $a^2=a$ for $a \in R$. Prove if $I$ is a prime ideal in $R$ then $R/I$ has 2 elements

6 votes

Surface of a 2-sphere expressed as union of two closed disks

6 votes

Irreducible $f(x) \in F[x]$ of prime degree, $E/F$ finite extension, $p \mid [E:F]$.

6 votes

Show $x^6 + 1.5x^5 + 3x - 4.5$ is irreducible in $\mathbb Q[x]$.

6 votes

Show group of order $4n + 2$ has a subgroup of index 2.

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