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A long time ago, I studied mathematics. I used to know three or four things about complex analysis, topological vector spaces, and point set topology. But I've forgotten at least five of them.


5h
reviewed Leave Open Is the map $T(u, v)=(A(u, v), v)$ surjective?
5h
reviewed Leave Open Integral of $\int \frac{x^2-4}{x^2\sqrt{4+x^2+x^4}} \,dx$
5h
reviewed Close What is the best math equation to find base components of a complex wave?
10h
comment Somewhat Confused about Notation in Category Theory
Yes, it should be $\phi^{-1}(\operatorname{id}_{S\cup T})$.
11h
reviewed Close I need to model a population where every component have a fixed life span.
12h
reviewed Close How to solve the following equation by extracting square roots?
12h
reviewed Looks OK Does de Rham theorem hold for manifolds with boundary?
12h
answered Why is the Gelfand transform injective?
15h
comment Is there any proof for this formula $\lim_{n \to ∞} \prod_{k=1}^n \left (1+\dfrac {kx}{n^2} \right) =e^{x⁄2}$
Hint: take logarithms. Use the Taylor expansion of $\log (1+z)$.
15h
comment Basic question about similar matrices
More direct, $A$ and $B$ being similar means by definition that there is a $P$ with $B = PAP^{-1}$.
16h
comment Brainteaser: Player A has £1, Player B £99. They flip a coin. The loser pays the other £1. Expected number of games before one is bankrupt?
@Lord_Gestalter If player $A$ has 100 pounds, player $B$ is already bankrupt, meaning the game is over, $0$ more coin flips.
22h
awarded  operator-theory
1d
reviewed Close Probability of infinite intersections
1d
reviewed Leave Closed How many subsets of $\{1, 2, …, n\}$ contain $1$ and how many don't?
1d
reviewed Reopen gcd and lcm from prime factorization proof
1d
reviewed Leave Closed Is there any bound on the number of generators of a monomial ideal in C(x,y)?
1d
reviewed Leave Closed What are the properties of the category of all categories within itself?
1d
answered Convex interior topology
1d
comment Continuous bijection from $[0,1]$ to $[0,0.5)\cup (0.5,1]$
Can you find some topological property that one of the spaces has but the other hasn't?
1d
comment Dense subset of the plane
$U$ is dense because $U\supset \mathbb{Q}\times\mathbb{Q}$ indeed. But that implies $\operatorname{diam}(U) \geqslant \operatorname{diam}(\mathbb{Q}\times\mathbb{Q}) = +\infty$. Did you mean measure, perhaps, instead of diameter?