How do I boot into an older kernel? I install new kernel (from launchpad mainline kernel) in my Ubuntu 12.04, in order to test some issues I'm having.  Now, older kernels don't appear in the list at boot, just the new kernel. I can boot from the old kernel changing boot parameters at grub time, but I rather like to have a list entry I can select.
How can I boot with an older kernel version? The latest kernel is causing problems with my sound, which worked fine with an older version. As I have only Ubuntu installed, Grub is not getting displayed while booting. How can I manually choose my kernel version while booting?
Need to understand why my Schengen visa was refused Do you know a good way to compare PDF files side-by-side and show the modifications between the two?  I'm looking for Windows software to accomplish this. It would be great if you can post both free and not-free products. "ERROR: Extra }, or forgotten \right." in align* enviroment Why do I get a "ERROR: Extra }, or forgotten \right." in the following align* environment?  \begin{align*}     \int\left[\left(\der{E_z}{y}-\der{E_y}{z}-\mu\der{\alpha}{t}\right)\alpha&amp;+\left(\der{E_x}{z}-\der{E_x}{x}-\mu\der{\beta}{t}\right)\beta\\     &amp;+\left(\der{E_y}{x}-\der{E_x}{y}-\mu\der{\gamma}{t}\right)\gamma\left.\vphantom{\left(\der{E_y}{x}}\right]d\omega=0.     \end{align*}   thanks Did J.R.R. Tolkien ever write a deeper connection between The Hobbit & Lord of the Rings: Fellowship of the ring? There seems to be a bit of a gap between The Hobbit and Lord of the Rings: Fellowship of the Ring. While we know the outcomes from The Hobbit to Lord of the Rings, we don't know what happened between them. Did Tolkien ever novelize this?  Connecting the two movies together, I've pieced the following sub-plots:   Saruman becomes corrupted by peering into the Palantír of Orthanc at some point after The Hobbit Gollum gets captured by Sauron's army Legolas leaves his homeland, in search of a ranger of some importance Tauriel is banished from her homeland Bilbo returns to the Shire, and finally relinquishes the one ring on his 111th birthday to Frodo   What doesn't seemed joined by the books or the two trilogy films:   How did a powerful white wizard like Saruman become enchanted/corrupted by Sauron What was Gollum doing before he was captured Where was Thranduil during the events of Lord of the Rings What happened to Tauriel during Lord of the Rings What happened to the remaining company of dwarves after The Hobbit Who now resides within Erebor What happened to Radagast during Lord of the Rings Is it theoretically possible that every person in this village has a different jar of coins? A village has about $1$ million residents. Suppose each resident has a jar with $100$ coins in it.Two jars are considered to be “equivalent” if they have the same number of 10c, 20c, 50c, $\$$1 and $\$$2 coins in them. How many different (i.e. nonequivalent) jars of coins exist? Is it theoretically possible that every person in this village has a different jar of coins?  I understood that if we have $k_1$ 10 coins in the jar where $k_1$ is between $0$ and $100$ inclusive, then there are $100-k_1$ coins of other denominations in the jar. Likewise, we also have $k_2$ 20c coins in the jar where $k_2$ is between $0$ and $100$ inclusive, so the combined number of 50c, $\$$1, and $\$$2 coins in the jar is $100-k_1-k_2$. And we always have some $k_1, k_2, k_3, k_4, k_5$, let's say, so that the sum of those $k$'s is $100$.  I got stuck since I don't know how to 'compare' the number of coins in jars for different residents. Transcendental numbers and irrationality measure Where are the default settings saved for the Command Prompt defaults?  Are they stored in the registry somewhere, or in a file?  I would love to make a .reg file to set my default preferences when logging into a machine for the first time.  I'm talking about the default application settings: QuickEdit mode, buffer size, text color, etc. Why use apparently meaningless do-while and if-else statements in macros? I have a dual boot laptop (Windows 10). I ran general updates and now my Ubuntu won't boot.   I ran in recovery mode and still it freezes in the same place.      Kernel panic – not syncing: Attempted to kill init! exit code=0x00007f00 -I'm not a religious person so this is not a denial. I'm just trying to understand the most fundamental topic about Universe.  -I know the Big Bang cosmological model is not a law but it's a theory.  -I'm a non-Native English speaker so please let me know if the question is not clear for you.  -This question came to my mind after watching Real Analysis Riemann Integration - Strict Monotonicity for Integrals If $f,g$ are Riemann integrable on $[a,b]$, and $f(x) &lt; g(x)$ for all $x \in [a,b]$, prove that $$ \int_a^b f(x) \,dx &lt; \int_a^b g(x) \,dx$$  This is a strict inequality. I know how to prove the monotonicity of integrals with the non strict inequality, but I am not sure how to do this. Is past simple ever justified with 'by'? Can anyone shed some light on the following aspect?  By 5 PM I gave him the book.  By 5 PM I had given him the book.  In both the sentences the book was given before 5 PM and no later than 5 PM. But I can't understand then what the difference between them is. a Also is it right to use simple past with 'by' at all? If yes, then why and if not, then why is it incorrect to use simple past with 'by'? ZIP Code (US Postal Code) validation I thought people would be working on little code projects together, but I don't see them, so here's an easy one:  Code that validates a valid US Zip Code. I know there are ZIP code databases out there, but there are still uses, like web pages, quick validation, and also the fact that zip codes keep getting issued, so you might want to use weak validation.  I wrote a little bit about zip codes in a side project on my wiki/blog:    There is also a new, weird type of zip code.     I can do the javascript code, but it would be interesting to see how many languages we can get here. I'm new to Linux and Ubuntu. People keep using ~/filename in their answers to my questions. What does ~/ mean? Which derivation of drift velocity is correct? In the derivation of drift velocity I have seen two variations and want to know which one's correct.   $s=ut+\frac{at^2}{2}$ Assume that the drift velocity of any electron in any conductor is : $$\vec{v_d}=\frac{\vec{l}}{t}$$ Due to the electric field the acceleration of electrons in any conductor is: $$a=\frac{-e\vec{E}}{m}$$ Now the distance travelled by an electron after a long time (initial thermal velocity = 0) $$l=\frac{at^2}{2}\implies \frac{-e\vec{E}t^2}{2m}$$ the time between the collisions is $\tau$ $\therefore$ $$l=\frac{-e\vec{E}\tau^2}{2m}$$ thus the velocity is $$\vec{v_d}=\frac{-e\vec{E}\tau}{2m}$$ In another proof I saw the author using $v=u+at \implies v_d=\frac{-e\vec{E}\tau}{m}$   My question is which of the the two equations of motion can be used in the proof? Can they be used at all. Folders Completely Hidden Except via Save As I'm using an audio program called Supercollider on Windows 10. I have saved some files in a folder called "tunes" from SC via Save As, but Windows Explorer and CMD both seem unable to recognise access or recognise the "tunes" folder.  C:\Users\HP&gt;cd "C:\Program Files (x86)\SuperCollider-3.7.2\supercollider-robin\tunes" The system cannot find the path specified.  C:\Program Files (x86)\SuperCollider-3.7.2\supercollider-robin&gt;dir/a doesn't show it either.  However, when saving from SC with Save As, it's there clear as day. I can even shift+right-click to get the path: "C:\Program Files (x86)\SuperCollider-3.7.2\supercollider-robin\tunes"  This makes no sense to me. Can anyone explain and tell me how to access my folder tunes folder please? How can faces be rotated around their individual normals? In edit mode with these settings:   Face selection Pivot point: individual origins Transformation orientation: Normal   The faces don't rotate around the normals, is there another required setting that I overlooked? 8 bits representing the number 7 look like this:  00000111   Three bits are set.     What are algorithms to determine the number of set bits in a 32-bit integer? How Do I Allow A Standard User to Install Programs? Yes, I realize that defeats the purpose of standard user. But here is the situation. A friend of mine wants to set up time limits for his home schooled son, and his son needs to be a standard user in order to use Family Safety. But at the same time, his son (who I personally think is way too old to have time limits), needs to be able to update and install some of his games on his computer. So I guess it's just sort of "light" parental control (just something to help him focus and not get distracted with hours of gaming).  It is the basic version of Windows 8.1, so there is no Group Policy editor (gpedit.msc). However, I'm fairly good with computers and I figured out how to install it. But it doesn't have a lot of the features that the "real" gpedit seems to have. Here is an example of what is in my gpedit: I am trying to prove this inequality  $$|e^z-1|\leq e^{|z|}-1\leq |z|e^{|z|}$$  I've tried calculating the difference in their power series   $$\sum_{k=0}^\infty\frac{|z|^k}{k!}-1-\left|\,\sum_{k=0}^\infty\frac{z^k}{k!}-1\,\right|$$   but did not get anywhere.