A compact operator T such that $T^{2}=T$ is a finite rank operator
An idempotent operator is compact if and only if it is of finite rank
How to create a sub array from another array in Java? I have a number of friends on my battle.net account. However, sometimes I don't want to appear online to some (or all) of them. Is there a way to be hidden from your friends on battle.net? How do I hide Thunderbird from the launcher while running? I have a question for you all.  It's possible to "hide" thunderbird from the unity application bar when thunderbird is running?  For my is unhelpfull see thunderbird on the "mail" tray ( near shutdown button) and the unity "running" application.  If i close thunder with the close button, it don't notify me new email. Application of Kirchhoff's laws in circuits with inductors As we know, the  are applicable for conservative electric fields. Now it is applicable for circuits where inductors are present but the field there is not .  So how are the  Kirchhoff's laws  applicable there? How to resolve ambiguous column names when retrieving results? I have two tables in my database:  NEWS table with columns:   id - the news id  user - the user id of the author)   USERS table with columns:   id - the user id   I want to execute this SQL:   SELECT * FROM news JOIN users ON news.user = user.id    When I get the results in PHP I would like to get associative array and get column names by $row['column-name']. How do I get the news ID and the user ID, having the same column name? Where can I ask questions about the computer problems I have been facing?  Current problems I am facing with my computers:   Windows 7 errors random "Blue screens of Death" computer randomly freezing up Xfire challenges What are the technical obstacles that doesn't allow OSX to be installed in a PC? $M + N$ and $M \cap N$ are finitely generated $A$-modules implies $M$ and $N$ finitely generated using exact sequences Adding Foreign Key Error I´m trying to model a race track using a bezier curve, but, when I add modifiers -> array -> curve I get this :    What should I check for to get the mesh following the curve correctly ? Thankyou I am trying to understand the permission usage for the cp command in unix.   What are the permissions required for copying a file into a directory What are the permissions required for copying out of a directory?   I'm assuming for my first question, the directory will need execute permissions to perform the operation and the file will need read permissions to read the binary of the file and then the output file will need write permissions.  For the second the directory itself would need execute permissions to perform the copy operation and the file would need read permissions to extract the data from the file.  Can anyone tell me if my logic is flawed? Show that there is no epimorphism from ($\mathbb{Q}$, +) to ($\mathbb{Z}$, +) What happens to the semiconductor in an LED when it is given voltage that it cannot handle? I've found a strange thing: some Stack Exchange sites have their own domain names instead of https://[site name].stackexchange.com. The websites which have their own domain names are:          Are there any parameters for allocating specific URLs to Stack Exchange sites? How can I run Maniac Mansion through ScummVM? Day of the Tentacle Remastered Edition includes Maniac Mansion as a .  The game appears to be exactly the same as the old DOS release (albeit, the enhanced version).  If possible, how can I run this game through ScummVM?  Pointing ScummVM at my installed game doesn't work.  What do I need to do to play this version of Maniac Mansion? What causes the 'Connection Refused' message? This is a  about Connection Refused    We see a lot of questions to the effect      When I try to connect to a system I get a message      Connection refused      Why is this ? Moved bin and other folders! How to get them back? Proving an integral equation. How can I find the codes related to a function in GAP? When I use "??" in front of the name of function there is no help, so I want to find the code of function among packages. Expected value as a function of quantiles?