1,210 reputation
216
bio website andrew-christianson.github.io
location Los Angeles, CA
age 23
visits member for 2 years, 10 months
seen Jul 8 at 14:13

Dec
10
comment simple question on SVD
Sorry I should've been more clear - I was referring to the norm of the vector (which measures length) rather than the number of elements. You've figured out the solution I was suggesting yourself though!
Dec
10
comment simple question on SVD
Why not compare the length of the $v_{old}$ and $v$? Should be cheaper than SVD. Or am I missing something?
Sep
24
comment What is the probability of the box?
Definitely, but I think it's also useful to do some easy problems with more complicated machinery, just to see how things work.
Sep
24
comment How is $e^x$ read aloud?
Perhaps not in the first explanation of an equation, but if you have to say it many times, that's what I default to. Particularly reading/taking about anything in continuous time finance, $e^{-rT}$ quickly becomes 'ee minus arr tee' for me.
Sep
22
comment calculus of variations question
@Rut welcome to math.se! Posts of this nature should usually be comments on the main question rather than answers. The latter are generally reserved for complete solutions to the problem at hand.
Sep
22
comment Sum of Infinite Surds
I'm not quite sure I understand precisely the value you're computing. Is it: $$\sqrt{10+\sqrt{100+\sqrt{1000+\cdots}}}$$
Sep
21
comment Trig to determine distance: boat on course parallel to shore.
Draw it out first. Can you see how the boat's path in the problem forms a triangle?
Sep
18
comment Cubic equation - finding coefficients
I agree with the above comments that this is not at all the appropriate form for this question. That said, if I interpret your variable names correctly, you fit the parabola with equation $$ax^2+bx+c$$ through three points, $p_1=(x_1,y_1), p_2=(x_2,y_2), p_3=(x_3,y_3)$ and propose to do so with the equations $$ a=\frac{((y_2-y_1)(x_1-x_3))+((y_3-y_1)(x_2-x_2))}{(x_1-x_3)(x_2^2-x_1^2)+(x_2-x‌​_1)(x_3^2-x_1^2)}$$ $$ b= \frac{(y_2-y_1)-a(x_2^2-x_1^2)}{x_2-x_1} $$ $$c = y_1-ax_1^2 - bx_1 $$ and are asking for a similar equation to fit a cubic through three (or four?) points.
Sep
18
comment Intuition for the Product of Vector and Matrices: $x^TAx $
@Alex I know what matrix multiplication means, I'm asking for ways to perform it quickly, or at least 'know' categorical/broad stroke information about the products at a glance.
Sep
17
comment Is it possible to practice mental math too often?
George, what sort of arithmetic are you practicing? Is your question more about getting basic times tables down, or how to do more complicated 2+ digit multiplication/mulitiplication by fractions?
Sep
17
comment Need help with statistics homework
Alex, welcome to math.se! I've added the homework tag to your question. There's nothing wrong with asking homework questions on the site, as long as you mark them as such. If this is not homework, feel free to remove the tag.
Sep
8
comment How do these cross products work
I know links aren't always the most helpful, but I would refer you to: en.wikipedia.org/wiki/Cross_product#Algebraic_properties I often find it helpful to have the list of properties as reference when learning how to deal with a new operation (like the cross product).
Jun
19
comment Cutting cake into 5 equal pieces
@Steven if your read nothing else of this answer, note what Mark has to say in his final sentence. Absorbing this way of will be far more profitable for you in the long run than any direct answers to your questions.
Jun
8
comment Good in-depth books for precalc?
It's easier to help if you give us some idea of where you are currently. You've taken algebra and precalc, what do you remember and what do you want to review? You say the Everything Guide is not in-depth enough - what are your problems with it? Needs more step-by-step? Needs more proofs? Needs more examples? I'm not aware of a universally prescribed College Algebra book, so anything I or someone else recommend is based on your preferences.
Jun
8
comment Good in-depth books for precalc?
Perhaps you could give us a little bit of a better idea of where you think you are currently? I wouldn't want to insult you by offering too simplistic a book, nor stymy you by offering something too complicated. Looking over the outline for the Accuplacer test(collegeboard.com/student/testing/accuplacer/…), I see it covers everything from arithmetic through basic college algebra. Where do you think you fit on that spectrum, and which topics in particular do you feel confident or unsure about?
Jun
8
comment Multiple Regression over an experimental dataset
This might be better received over at stats.stackexchange.com . That said, we don't call it Multiple Linear Regression for nothing. Your data seems to oscillate significantly, at least based on the 3D Plot. If you could post a view of the data as a point cloud, it would be easier (for me at least) to visualize. Given how dispersed the data seems we wouldn't expect a linear model, even with an interaction term, to have a particularly high $R^2$. Where is this data drawn from?
Jun
5
comment Clearing a manufacturing backlog
To clarify, $X$ and $Y$ are the rates at which the eponymous stages process widgets, correct?
Jun
4
comment Ratio and number theory
@Kartik. Ah. I didn't know it was an exam problem. Marvis' answer is definitely the better solution then
Jun
3
comment Ratio and number theory
@Kartik I feel obliged to point out that this is problem where brute force can definitely prove the right answer. There are only 900 possible $K$s, so it is feasible to check each one to find the minimum.
Jun
3
comment Ratio and number theory
With positive division, floor is effectively a truncate function; it returns the integer part of a ratio. Lets take $x=152$ as an example. So let, $d_{100}(152)= \left\lfloor\frac{152}{100}\right\rfloor = \left\lfloor 1.52\right\rfloor = 1$ which is the hundreds digit. Then clearly, $\left\lfloor\frac{152-100*1}{10}\right\rfloor = 5$ is the tens digit, because we've subtracted off the hundreds. Just rephrase the $100*1$ to $100*d_{100}(152)$, and you've got a general expression for the tens digit. Repeat the same process for the 1s digit.