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Chessanator
  • Member for 7 years, 9 months
  • Last seen this week
19 votes
Accepted

What is the difference betwen equivalence and isomorphism of functors in categories.

12 votes
Accepted

Confused by proof of the irrationality of root 2: if $p^2$ is divisible by $2$, then so is $p$.

12 votes

How to prove that there exist no functions $f,g:\Bbb{R}\to\Bbb{R}$ such that $f(g(x))=x^{2018}$ and $g(f(x))=x^{2019}$?

11 votes

Show that $d(x,y) = \cos(x-y) + 3$ for $x \ne y$ is a metric.

10 votes
Accepted

Difference between internal hom and exponential objects?

8 votes
Accepted

Dual Map as Composition

8 votes
Accepted

Joins and Meets in Preorder

8 votes
Accepted

Is ${\bf Set}$ a Mal'cev category?

7 votes

Is the field of real numbers a unique factorization domain?

6 votes
Accepted

Why is the direct product $\mathbb{R}\times\mathbb{R}$ undefined when $\mathbb{R}$ is viewed as a field?

6 votes

Why $p$th power are not allowed in radical extensions?

6 votes
Accepted

Tensor product of $\mathbb{C}$ and $\mathbb{Q}$ is not zero

5 votes
Accepted

Find $a\in \mathbb{Z}_5[x] /I$ so $a^3=x^2+1+I$

5 votes
Accepted

Product of infinitesimal by inverse of non infinitesimal is infinitesimal

5 votes
Accepted

If one vertical arrow in a pullback is an iso, then so is the other

5 votes
Accepted

Why are simplicial sets contravariant functors and not covariant?

5 votes
Accepted

Vector spaces, matrices from $\mathbb R^{2\times 2}$

5 votes

Is $2x-y = 0$ a plane or line in 3d?

4 votes

Does there exist a homomorphism from $\mathbb{Z}$ to any ring?

4 votes
Accepted

Every nonempty subset of $\mathbb{N}$ has a smallest element.

4 votes
Accepted

$(a_n)$ is covergent. $(k_n)\in\mathbb{N}$, $(k_n)\to\infty$. Decide if $(a_{k_n})$ is covergent.

4 votes

Mistake in assuming every subring of $R \times S$ is equal to some $A \times B$ for some $A\subset R, B\subset S$

4 votes
Accepted

Simplifying $\sqrt{xy\mathstrut}\sqrt{x^3y}$ - two different paths, different results

4 votes
Accepted

Category of real numbers whose categorical sums/products are regular sums/products

4 votes

When does a map induce a structure in a concrete category?

4 votes
Accepted

Why are these two quotient spaces not homeomorphic?

4 votes
Accepted

There exists a smallest linear subspace that contains every $M\in\mathcal{M}$

4 votes

Finding the nth root of complex number. Example z=(1-i)^1/5

4 votes

Prove that $|\{1,2,3,...,n\}|=n$ for all natural numbers $n$ by mathematical induction

4 votes

Why the empty set isn't a terminal object in the $\mathcal{SET}$ as well as it is an initial object?

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