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2016-07-26 16:03:08 -0600 commented answer check if symbolic expression contains a variable

Is there an extension of this to functions? For example,

y(x) = -3/(x^3 + 3*c + 3*x) + f(c)

? I am aware that this does not work as written, but I wonder if it is possible to extend it.

2016-07-26 15:42:46 -0600 received badge  Editor (source)
2016-07-26 15:41:17 -0600 asked a question Detecting highest order derivative of a function in an equation

I am working with some code that generates an equation with a number of derivatives. I do not know the order of the derivatives or the order. If I was working with something algebraic, for example:

a, b, c = var('a,b,c')
eqn = a**2 + 3*b + c

I could use something like

coeffs = eqn.coefficients(a)
highest_order = coeffs[-1][1]

to find out the highest order of a. It would also be okay if i could figure out the order of every derivative of a given function in an equation. I can do this if I know before-hand what the order of the derivative is

eqn += f(a,b,c).diff(a,b)

and see that in fact that derivative is there somewhere. But what I really want, is for

eqn = f(a,b,c).diff(a,b) + b*f(a,b,c).diff(b,3) + c

to have something that (a) tells me 'eqn has [D[0,1](f)(a,b,c), D[1,1,1](f)(a,b,c)]' or, (b) 'the highest order derivative of eqn is D[1,1,1](f)(a,b,c)'.

2016-07-22 13:09:26 -0600 commented answer What is broken on this site

Yes. After someone replied to the post I linked above I was bumped to 11 karma and was able to submit the answer to the original question I wanted to answer. I realize this is not a problem for me now, but it is a hindrance to new users.

2016-07-22 13:07:13 -0600 commented question Posting answers as a new user

Is there some indication that an answer has been submitted and is up for approval? I did not receive such a notice. I think seeing that would have comforted me.

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2016-07-20 02:22:58 -0600 received badge  Teacher (source)
2016-07-19 14:41:02 -0600 answered a question What is broken on this site

I recently joined the site to post an answer to something that I realized I could help with. However, I soon went on to discover that one needs at least 10 karma to answer. I can see how there is a need to eliminate spam, but I think the site could benefit from some other form of verification so as to not scare off new users.

2016-07-19 14:37:00 -0600 commented question Posting answers as a new user

This is fixed now that someone up-voted this post. Therefore it appears that you need at least 10 karma to answer a question. I think this is something that should be fixed.

2016-07-19 14:35:41 -0600 answered a question Pattern matching in differential equations

Wildcards might help you here. For instance, I got this (not ideal) way to produce what you want:

x,y,t = var('x,y,t')
f = function('f'); g = function('g')
w0 = SR.wild(0); w1 = SR.wild(1)
pattern1 = w0*f(t).diff(t,2)(t = w0); pattern2 = w0*f(t).diff(t,2)(t = -w0)
eqn = f(x*y) + f(x*(1-y)) + f((1-x)*y) + f((1-x)*(1-y)) - f(x) - f(1-x) - f(y) -f(1-y)
eqn2d = eqn.diff(x,y)
eqn2d.subs( pattern1 == g(w0) - f(t).diff(t)(t=w0), pattern2 == g(-w0) + f(t).diff(t)(t=-w0) )

Now obviously it would be nice if Sage could just notice that pattern1 is related to pattern2, but I couldn't get it to work out (maybe someone better at Sage can?). It is worth noting that I also tried using

pattern = f(t).diff(t)(t = w0) + w0*f(t).diff(t,2)(t = w0)

but eqn2d seems to have no occurrences of that pattern: eqn2d.has(pattern) returns False. I would be curious to see if anyone has an explanation for this as well.

2016-07-19 04:56:09 -0600 received badge  Student (source)
2016-07-18 13:18:45 -0600 commented answer Posting answers as a new user

All that appears to have changed is that I do not need to enter a captcha now, my post is still not submitting. Maybe it is relevant that the post is a wiki?

I also wonder if this issue can be placed on the FAQ or another place where new users can easily find this information.

2016-07-18 13:02:54 -0600 asked a question Posting answers as a new user

I am a newcomer to I have found a question which I know how to answer but when I try to submit the page simply refreshes. I am guessing this has something to do with having low karma but am unsure how to increase my karma since I do not have any questions at this time. I found a similar unreplied to question (to which I apparently cannot link because my karma is insufficient?) on askbot but the solution there of adding a comment first does not work for me. Any ideas on how to fix this are appreciated.