What is an example of a transparent matter with refractive index less than 1?
Medium with refractive index less than unity?
Given a group $ G $, how many topological/Lie group structures does $ G $ have? Why I can't scan the WhatsApp QR? Why I can't scan the Whatsapp QR?   I reinstalled WhatsApp  I already have the WhatsApp Web Option  I put it to scan but it just doesn't scan it. I Installed a QR Scanner app and it works perfectly. I uninstalled the QR Scanner app to see if that was the issue and still doesn't work.   My phone is a Samsung Galaxy Ace. "who" vs. "whom" and when to omit the relative pronoun This is the boy who was asked by the teacher to stay back after school. This is the boy whom the teacher asked to stay back after school. This is the boy the teacher asked to stay back after school.   Are the sentences above correct ? Python and regular expression with Unicode I need to delete some Unicode symbols from the string 'بِسْمِ اللَّهِ الرَّحْمَٰنِ الرَّحِيمِ'  I know they exist here for sure. I tried:  re.sub('([\u064B-\u0652\u06D4\u0670\u0674\u06D5-\u06ED]+)', '', 'بِسْمِ اللَّهِ الرَّحْمَٰنِ الرَّحِيمِ')   but it doesn't work. String stays the same. What am I doing wrong? $\dfrac{pq}{r}+\dfrac{qr}{p}+\dfrac{pr}{q}\in\Bbb Z\,\Rightarrow\, \dfrac{pq}{r},\dfrac{qr}{p},\dfrac{pr}{q}\in\Bbb Z$ Let $p,q,r$ be integers such that the symmetric sum of fractions $\dfrac{pq}{r}+\dfrac{qr}{p}+\dfrac{pr}{q}$ is an integer. Prove that each of the numbers: $\dfrac{pq}{r},\dfrac{qr}{p},\dfrac{pr}{q}$ is an integer. How to do this? Someone posted but deleted soon. How do I craft command blocks? I went to my friends house, and he was playing Minecraft on the PC. He showed me a command block, but when I went home and opened Minecraft pocket edition, I went to the crafting table but I could not find any command blocks to craft.  What are command blocks and how do I craft them? How to reinstall kernels after accidental deletion with a separate boot partition and an encrypted LVM volume? My installation is a Ubuntu 12.04 64 bits (Classic "legacy" MBR - No  EFI/UEFI here) with two partition :   A separate boot partition An encrypted LUKS volume that contains the system Logical Volumes   I seems I accidentaly deleted all the current kernel files with:  sudo apt-get purge 'linux-image-3.8*'    I applied the updates with Ubuntu Software Update, rebooted and then: Ubuntu can't boot.  I tried to boot on a live DVD with a 14.04.1 Ubuntu and can still see my partitions (described later):  /dev/sda1 (boot) /dev/sda2 (Encrypted volume) /dev/sda5 (Logical Volume container)  What are the steps to recover/reinstall the kernel files and dependencies in order to cleanly reboot my installed system?  Thanks in advance Match the size of multiple background images In  tutorial in the first couple of minutes it is explained how to set the dimensions of a cube and match the background images to this mesh. I have tried to get it right but till now I did not succeed. This is what the creator of the tutorial says in the comments:      I'm keeping only one mesh and just changing the x and y values of my   background images you can see me add at 3:00. We want all the   background images to match that one mesh.   Here are my images:     I think front and back images match to the mesh. But for the right, left and top view I only get one dimension matching. I don't see how you can match both dimensions matching without changing the mesh Do individual rays of light lose energy via the inverse square law? How can i make the grass bend when this cube hits it?  Here's the picture: Teen fantasy book series about a world where people who showed creativity could learn magic, and were killed for it Can black holes form in a finite amount of time? What are the differences between "su", "sudo -s", "sudo -i", "sudo su"? I already read it from manual but I can't see difference..  su - change user ID or become superuser  sudo -s [command]   The -s (shell) option runs the shell specified by the SHELL environment variable if it is set or the shell as specified in passwd(5).  If a                    command is specified, it is passed to the shell for execution.  Otherwise, an interactive shell is executed.  sudo -i disappear description in manual What do bookcases do for enchanting? can I use same functions with the different type of argument inputs Prove that a graph with the same number of edges and vertices contains one cycle In the sentence below, are the usages of apostrophes correct?     In research about parent stereotypes, the correlation between parents’ beliefs about their early elementary age children’s’ mathematics abilities and the children’s actual abilities increased as children aged. Is exon order always preserved in splicing? Are there any cases in which the splicing machinery constructs an mRNA in which the exons are not in the 5' -> 3' genomic order? I'm interested any such cases, whether they involve constitutive or alternative splicing. I am on a multiplayer Minecraft realm in 1.16 and I was wondering if there was any way to set a chest's Items to a player's entire Inventory including the offhand and armor slots. Using data modify &lt;chest coordinates&gt; Items set from entity &lt;player&gt; Inventory, I was only able to copy over the hotbar and the top two rows of a player's inventory. I honestly don't care if it takes 50 command blocks, any response is gladly appreciated. Can a shadow move faster than the speed of light?