Timeline for BUG: Why is Series[] getting this expression wrong?
Current License: CC BY-SA 4.0
19 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| Jan 14, 2024 at 13:51 | comment | added | Jerry Guern | @DanielLichtblau Wolfram got back to me and confirmed it's an issue with Series[]. Btw, I missed your joke on the first read. LOL | |
| Jan 14, 2024 at 13:49 | history | edited | Jerry Guern | CC BY-SA 4.0 | Updated with new info from Wolfram |
| Jan 9, 2024 at 0:47 | comment | added | Daniel Lichtblau | I agree this is an unexpected mismatch. I'll look into making Limit get the weaker result too. (Okay, no, I'll look harder at Series. Maybe I missed something obvious.) | |
| Jan 8, 2024 at 23:58 | comment | added | Jerry Guern | @DanielLichtblau I see. I sent a note to Support though. The fact that Limit gets it but Series doesn't is definitely a software fail even if it's not a full on bug. | |
| Jan 7, 2024 at 21:01 | comment | added | Daniel Lichtblau | I don’t regard it as a bug. Conditional zeros that happen to always be zero are notoriously difficult to detect. (Usual cause is parametrized branch cuts that manage to cancel. They never want to inform anyone of the cancellation.) | |
| Jan 7, 2024 at 19:38 | comment | added | Jerry Guern | @DanielLichtblau Ah. So, is this a bug I should report to Wolfram? | |
| Jan 6, 2024 at 18:45 | comment | added | Daniel Lichtblau | I see what happened. I was using your f0, not mdel. As best I can tell, what's going amiss is that Series is giving a hidden zero for its constant term. It is not able to unwind that the value is zero independent of theta and q. | |
| Jan 6, 2024 at 4:49 | comment | added | Daniel Lichtblau | I’ll check again when I’m at my desk. Maybe I lost a factor of theta somewhere. | |
| Jan 6, 2024 at 0:28 | comment | added | Jerry Guern | @DanielLichtblau Limit[D[mdel, theta], theta -> 0] gives me 0, the correct answer. You can also plug in small values and see that for small theta, mdel is proportional to theta^2. For example, mdel /. {q -> .1, theta -> .01} yeilds -.000249 while mdel /. {q -> .1, theta -> .001} yield -.00000249, both of which are very good matches to my 2nd order expansion 0 + 0 -(cot(q)/4)*theta^2 | |
| Jan 4, 2024 at 23:39 | comment | added | Daniel Lichtblau | I am unable to replicate the claim that the first derivative (for example) in theta vanishes at the origin. In[21]:= InputForm[D[mdel,theta]/.theta->0] Out[21]//InputForm= Cot[q]/8. And indeed this agrees with the result I get from Series. But I can get zero by setting q to Pi/2: InputForm[D[mdel/.q->Pi/2,theta]/.theta->0] Out[24]//InputForm= 0. This is why I raised that possibility in my prior comment. | |
| Jan 4, 2024 at 22:48 | comment | added | Jerry Guern | @DanielLichtblau Thanks for the reply. I get exactly what you get for mdel. But I'm expanding the Taylor series in theta not q. The 0th and 1st derivs of mdel in theta are both zero. The 2nd deriv of mdel wrt theta at theta==0 is -cot(q)/2. So the series in theta should simply be 0 + 0 + (-Cot[q]/4)*theta^2 + O(theta^3) | |
| Jan 3, 2024 at 21:45 | comment | added | Daniel Lichtblau | This is more complicated than it needs to be. Start with mdel. After running your code, I have this. mdel = 1/(1 + Sqrt[Csc[q + theta/2]*Sin[q - theta/2]]);. If your result is different, look for why that happens. Did you have one of the parameters set to a value? Perhaps you had q=Pi/4? If none of this is the case maybe people can look further. But I would not expect the first derivative to be free of q (I bring that up because zero is independent of q). | |
| Jan 2, 2024 at 16:28 | history | edited | Jerry Guern | CC BY-SA 4.0 | Clarifying the question again in response to comments |
| Jan 1, 2024 at 2:25 | history | became hot network question | |||
| Jan 1, 2024 at 0:37 | history | edited | Jerry Guern | CC BY-SA 4.0 | edited title |
| Jan 1, 2024 at 0:18 | history | edited | Jerry Guern | CC BY-SA 4.0 | Clarified question |
| Dec 31, 2023 at 19:25 | answer | added | Bob Hanlon | timeline score: 2 | |
| Dec 31, 2023 at 19:17 | history | edited | Jerry Guern | CC BY-SA 4.0 | clarified verbage |
| Dec 31, 2023 at 18:24 | history | asked | Jerry Guern | CC BY-SA 4.0 |