On Ubuntu 18:04 Bionic (installed as a Singularity container) I issue   apt-get install okular   and I get the message "E: Unable to locate package okular".  Why? Is okular not in the apt repositories any more? How do I install it then?
When I try to install any package through the command line, I get an error.   $ sudo apt-get install &lt;package&gt; Reading package lists... Done Building dependency tree        Reading state information... Done E: Unable to locate package &lt;package&gt;   Can anyone help me on this?
Is for open connected $U$ the set $U_\varepsilon$ for small $\varepsilon$ connected? Let for an open connected subset $U\subset \mathbb R^n$ and a number $\varepsilon &gt;0$: $$ U_\varepsilon=\{x\in U: dist (x, \partial U)&gt; \varepsilon \}. $$ Then  $U_\varepsilon$ is open but in general not connected.  But I suppose that for sufficiently small $\varepsilon &gt;0$ also  $U_\varepsilon$ is connected. How to prove its?  Edit. Let's assume that additionally $U$ is bounded. Is then $U_\varepsilon$ connected? I have read answers to questions like  but I couldn't find a general rule for using "you" or "yourself", "him" or "himself" that could explain things I see in some examples.  For example, I think that one normally say (according to examples I see in ):   You bring something with you. You have something in you. You have something on you. You pressed her to you.   But on the other hand one would say:   You draw attention to yourself. He killed himself. You can try it out for yourself.   As a general rule, it seems that the reflexive pronoun is used when the object is the same as the subject of the verb. But for prepositional phrases, it seems more complicated. Do some prepositions (like "with", "in", "on") always come with the accusative form of the pronoun, and some other (like "for") don't? What about "to", that seems to be followed by both in different circumstances? When do we still need the table and figure environments when we don't want the enclosed objects to float? The LaTeX Wikibook :     Please understand: you do not have to use floating tables. It only   looks more professional. If you want to place your tables where they   lie in your source code, do not use table at all! This is a very   common misunderstanding among newcomers.   LaTeX works with an intricate , but I often prefer immediate placement with the H placement specifier (enabled by the float package).  My understanding is that captions and table/figure numbering require the table/figure environments. Is this really so, or are there ways around that? What other reasons are there for using table/figure if we know that we don't want a particular object to float? I am pretty sure it shouldn't, but want to make sure that I am doing this correctly in a proposal that I am working on.  Should job titles like "Project Manager" ever be capitalized, and if so when?  In particular I am concerned about capitalization in sentences like this where the title refers to a specific person, not the role in general.     "The Project Manager will   evaluate the alternatives and offer a   solution."   I'm a lot more certain that it should NOT be capitalized in sentences like this.     "A project manager is the person who   manages the budget and deliverable for   a project." What is the plural of "cottage tariff"? I am searching for [free] software that can produce nice looking graphical models, e.g.     Any suggestions would be appreciated. Mapping/texturing inside of a box I'm trying to replace the content of a box in a video, I have used the tracking system to match the movement of the video box with the interior of the one rendered in Blender, but I am running trough what seems a simple thing but I can't manage to get done.   I want the outside faces of the box to be invisible while hiding the inside of the box where the front wall of the box hides it, and the inside of the box to be rendered where visible. To do that I thought it would be as simple as making the face normals point to the inside of the box and giving a color only to the faces where normals are pointing to. But I can't manage to get that, am I going the wrong road? How can I achieve my goal? What are SPF records, and how do I configure them? This is a  about setting up .     I have an office with many computers that share a single external ip (I'm unsure if the address is static or dynamic). Each computer connects to our mail server via IMAP using outlook. Email is sent and received by those computers, and some users send and receive email on their mobile phones as well.  I am using  to generate an SPF record and I'm unsure about some of the fields in the wizard, specifically:   Enter any other domains who may send or relay mail for this domain Enter any IP addresses in CIDR format for netblocks that originate or relay mail for this domain Enter any other hosts which can send or relay mail for this domain How stringent should SPF-aware MTA's treat this?   the first few questions i'm fairly certain about... hope i have given enough info. revoking apex managed sharing permission I'm using apex managed sharing to grant permissions very similar to how it is done in this example trigger:   However, I also need the ability to revoke or delete this access and having it revert to private. The reason for this is my understanding is that there are only two access options for managed sharing access 'read' or 'edit' with no 3rd option of 'none' or 'private'.  Reference:   I've tried adding the shares to a list and deleting the list and I'm getting the error: DELETE_FAILED, cannot delete owner or rule share rows,   I also noticed there is a field on the share object 'isDeleted' this field however is not writable.  Does anyone have experience revoking or deleting these permissions once they've been created?  Thanks! How does the new Tie-Breaker work? In Clash of Clans there was an update on 9/16 adding a Tie-breaker into the wars, can anybody explain in detail how this mechanism works? One thing I've noticed about the relatively new sitewide layout is that I can't find a quick way to get back to www.stackexchange.com without finding StackExchange in the sites list. As a result, I am typing stackexchange in my toolbar a whole lot more often now. In particular, I tend to enjoy browsing the hot questions list and my own personal feed quite often so visit these pages on a daily basis. Is there an easy link to them that I'm missing?  (On the android app, there is a feed in the menu that I really appreciate, something like that on the desktop would be ideal!) Small snap loop devices visible in gnome-disk-utility OR what is the function of snap ubuntu-core I just realized using gnome-disk-utility (3.18.3.1) that my 16.04 installation is featuring several small loop devices.  In detail:   78 MB Loop Device (/var/lib/snapd/sn_ntu-core_352.snap -> /dev/loop0) 76 MB Loop Device (/var/lib/snapd/sn_ntu-core_216.snap -> /dev/loop1) 79 MB Loop Device (/var/lib/snapd/sn_ntu-core_423.snap -> /dev/loop2) 705 KB Loop Device (/var/lib/snapd/snaps/tpad_12.snap -> /dev/loop3) 684 KB Loop Device (/var/lib/snapd/snaps/tpad_11.snap -> /dev/loop5) 705 KB Loop Device (/var/lib/snapd/snaps/tpad_13.snap -> /dev/loop6)   Image showing gnome-disk-utility    My questions are as follows:   What are those loop devices used for? Is that an expected behaviour in 16.04? Can  I remove them? And if so - how (just by unmounting)? Is there any risk in removing the 2 installed snaps mentioned below (tpad and ubuntu-core)   UPDATE  Installed snaps  According to snap list I do have   tpad (1.8.1) ubuntu-core (16.04.1)   installed. I remember I did tinker with snaps for a short time when  I realized this option - but am pretty unsure if  I  installed those 2 snaps - or if they are pre-/auto-installed.  tpad is a Terminal text editor with GUI-like user interface  I  did install manually, as  I don't use it  I removed it now via sudo snap remove tpad. As a result the 2 loop devices pointing to tpad are gone in gnome-disk-utility.  So - at the current point  I am still having 1 snap installed called ubuntu-core and I don't know what it is used for. The summary of this particular snap is   The ubuntu-core OS snap Which SUSY models are affected by the recent LHCb result? The LHCb has recently  the observation of $B_s \rightarrow \mu^+ \mu^-$ with a branching ratio that agrees with the Standard Model (SM). There are many blog posts about it (See: ).  The result initiated a debate over its implication on Supersymmetry in general, which I don't want to discuss here. But it certainly affects some Supersymmetric models that may have a different prediction than that of the SM.   My questions are,   Which models in particular are affected (or variants of models)? What is the MSSM status? The NMSSM? Does the result affect some of the parameter space of a particular model, or does it rule out an entire model/or variant of a model? Integration by partial fractions; how and why does it work? I have a python script that checks file path names for corresponding files in a drive location. All of the inputs were originally hard coded, and it works well enough:      import arcpy     import os     shp = r"C:\Users\dotwell\Desktop\Locations.shp"     fields = ["ZdriveLoc"]     f = open (r"C:\Users\dotwell\Desktop\ValMapsTest.txt", "w")     cursor = arcpy.da.SearchCursor (shp, fields)         for row in cursor:             Roll=str(row[0])             if not os.path.exists(Roll):             f.write(Roll + os.linesep)       f.close()   I'm trying to create an ArcGIS script tool from this, and I've only had partial success. I can use   shp = arcpy.GetParameterAsText(0)   to allow the user to select the shapefile. I'd like to then have a drop down list of the fields in that shapefile for the user to select the field that contains the path names, but I can't get it to work. Can someone show me how to generate the drop down? Here's my latest try. It works, but requires the user to type in the field name. It would also be nice for the user to designate the output text file name and location.   import arcpy import os shapefile = arcpy.GetParameterAsText(0) field = arcpy.GetParameterAsText(1) f = open (r"C:\Users\dotwell\Desktop\ValMapsTemp.txt", "w") cursor = arcpy.da.SearchCursor (shapefile, field) for row in cursor:     Roll=str(row[0])     if not os.path.exists(Roll):         f.write(Roll + os.linesep)   f.close()   Here are the parameters in my ArcGIS script tool: Is there a way to force a line break inside a math mode text?  My example:  \[   \underbrace{....}_\text{Some long text that should be multiline} \]   Trying  \text{Some long text that\\ should be multiline}   Didn't do the trick. How do you clone an Array of Objects in Javascript? Alternative to Photoshop or Paint.NET I have been moving toward Ubuntu from a long time Windows development background. The one program I cannot seem to do without is a graphic editor. I have seen recommendations for programs, but they turn out to be directed at children or tailored to working with personal photographs.  I am looking for something more for programming tasks like analysing colors, resizing, creating web graphics, etc. I have used Photoshop in the past and more recently have mostly used Paint.net for Windows.  Is there a program for Ubuntu that covers this area? Are $(\mathbb{R},+)$ and $(\mathbb{C},+)$ isomorphic as additive groups?  I know that there is a bijection between $\mathbb{R}$  and $\mathbb{C}$, and  asks whether they are isomorphic as abelian groups, are they referring to the additive abelian group? If so is there any simple isomorphism I can find? I know nothing about Hamel basis. Thanks. Constant Amortized Time What is meant by "Constant Amortized Time" when talking about time complexity of an algorithm?