Mathematics

12:25 PM

Hello.

Only 2.7k messages per week, on average :O.

I think there used to be upwards of 15k per week, on average, back in 2015.

Surely there are more math students on the internet though? Maybe they have went elsewhere.

Leaky Nun

12:42 PM

regression to the mean

Manolis Lyviakis

how to determine is a curve in R^2

is a closed loop

if*

Thorgott

check the images of the endpoints

1:42 PM

@JackDon It's an inside joke understood only by fans of Brian Lumley and the Necroscope himself. But I would say Harry used his higher maths to trap the math students in the void.

@JackDon Ignore that I couldn't resist. I'm just here cause I am terrible at math and I seek a numbers wizard.

1:56 PM

Is anti-social on your page meant to be asocial? @GWarner

I think most people that use anti-social actually mean asocial, that's what I ask.

Mats Granvik

Numbers are not difficult. It is all about counting the numbers, connecting the dots, and coloring in.

That's drawing numbers Mats :P.

Mats Granvik

What?

It is?

Connecting the dots, and colouring in.

Bad joke apparently :P.

@JackDon True, but it was necessary to use a more widely understood meaning.

I like connecting the dots and coloring in...

Mats Granvik

2:05 PM

180

@JackDon Harry is the protagonist in a series of books about a guy that can speak to the dead, hence learning math from past geniuses (Mobius especially) to travel through space and time to hunt vampires.

CroCo

if we know the highlighted angle to be 90-\theta, why how the rest two angles are established and why in this order?

Anyway, I was looking for help to figure out the progression of degrees to turn and how often based on speed to cover a grid of squares of a set size. I realize I could go in a linear fashion, turn 90 degrees for one grid and then 90 again. . im looking to spiral outward instead

4:45 PM

Suppose $A_{i}$, $B_{i}$, $C_{i}$ are chain complexes such that the sequence $0 \rightarrow A_i \rightarrow^{f} B_i \rightarrow^{g} C_i \rightarrow 0$ is exact for all i. Does this imply that $f,g$ are chain maps?

I would expect it to be false in general, but because of my lack of examples of chain complexes I couldnt find one

Mike Miller

That property has nothing to do with the differential, so certainly not

As a quick example, take $B_0 = B_1 = \Bbb Z$, with differential $d(b_1) = b_0$, with $A_1 = \Bbb Z$ and $C_0 = \Bbb Z$. Let $f$ be the obvious inclusion and $g$ be the obvious projection

But $d f(a_1) = db_1 = b_0 \neq 0 = f(0) = f(da_1)$, and $g(db_1) = g(b_0) = c_0 \neq 0 = d(0) = dg(b_1)$

So neither $f$ nor $g$ are chain maps in this example

This doesn't seem like what you wanted to ask, is it?

5:04 PM

@Mike This is actually what I wanted to ask. Actually I am proving that if such a short exact sequence exists then it induces a long exact sequence of the homology groups. But you have the map $H_p(A_i) \rightarrow^{f*} H_p(B_i)$ in the long exact sequence which can only happen if $f$ is a chain map, but my notes don't mention f being a chain map.

Mike Miller

Your notes are just being sloppy

$f$ is a chain map

Guessed so, thanks!