At our homes and utilities, the frequency is 50/60 Hz. Why not use any other value such as 500 Hz, 200 Hz or a small value 40 Hz, 30 Hz. Previously I searched it over the StackExchange but I want to know the technical and logical reason.
It just struck me that Utility Companies world-wide supply domestic users at either 50/60Hz. It could have been selected to be 10 Hz, or 100 Hz ... whatever  Was 50/60 chosen at random? Is there some rationale involved (efficiency, easier to construct/control etc) in restricting domestic supply to either 50/60 Hz?  In other words, if I were designing a power system, why would I pick 50/60 Hz instead of some other frequency?
Is there a software in Ubuntu that can analyse the cross references (theorems, lemmas, equations, etc.) in a tex project? There are some software to analyse the cross references among codes (Java or C++). I used to use a software for code analysis under Windows many years before. But I can't remember the name. On linux, an example might be LXR Cross Referencer.   Is there a similar tool that can analyse the latex files to find out the cross references of theorems, lemmas, propositions and equations? How to make child process die after parent exits? Suppose I have a process which spawns exactly one child process. Now when the parent process exits for whatever reason (normally or abnormally, by kill, ^C, assert failure or anything else) I want the child process to die. How to do that correctly?    Some similar question on stackoverflow:   (asked earlier)  (asked later)      Some similar question on stackoverflow for Windows: I've heard the number 7% thrown around a bunch of times, but I am struggling to find reputable data to back it up.   All I found was a quote from Warren Buffet that I couldn't find an original source for and data assembled by individuals on their personal websites. Is there something from an organization such as Market Watch making a statement about this?  I'm hoping to specifically find what the 10 or 20 year return is for investing in the S&amp;P 500 index fund or another diversified portfolio option. Left and right subscript / superscript I am trying to put two subscripts at the left and right of a character. For example, something like:  _{t} p_{x}  where p is in the middle. How do you do this? Export nodes from Drupal 4? Who are the diamond moderators, and what is their role? How to determine whether a given Linux is 32 bit or 64 bit? 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I started to write a loop to do it instead, but I can't shake the feeling that this must be possible in a single command. I could be wrong. Any ideas? Similar matrices and field extensions I finally got fed up when wanting to read about bash's read and it's -s option with man bash. I found the right spot eventually (around line 4500), but it was a frustrating as usual, since both /read and even /\s-s\s searches have way too many matches.  So, the question is: How can I read long man pages efficiently, or get same information in other ways, locally? As a specific example, how to reach the relevant documentation after seeing read -s pwd in a shell script? A good answer could be a shell script snippet, or hint about some tool and how it is used, or something else entirely, as long as it helps in finding the right spot to read.  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But I don't understand how to address the SECONDARY from stored procedure if I'm using distributed query. Is there a way to set some parameters to the query (like WITH statements or something), so query would use only replica and not the PRIMARY node?  The thing is, obviously from the start server1 is PRIMARY and server2 is SECONDARY, but when server1 fails then server2 is PRIMARY and server1 will be SECONDARY after it get fixed. So I cannot just use the static server2's name.  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After upgrading to El Capitan, I've started noticing annoying connections from the AirPlayXPCHelper to 192.168.1.0/24 subnet (my home network isn't in 192.168.1.0/24 subnet, it's in 172.16.0.0/12 subnet).  Questions are:   Why AirPlayXPCHelper goes out to this network on port 5000 and 7000? Is there a way to limit it's desire to go there?   Thank you. How can I prevent false score reports to global highscore tables? Browser and mobile games commonly have global highscore tables. It's also common for those tables to contain scores of 2,147,483,647 &mdash; where people have figured out the webservice call that reports scores and used it to record a fictitious score.  For simple puzzle games, we can defend against this by including a record of every move the player made (and any random seeds used to generate the level) with the score-reporting call. The entire game can then be reproduced and verified on the server.  However, this quickly becomes infeasible for anything larger than Pac-man.  How else can cheating of this kind be prevented? Derived template-class access to base-class member-data I can't figure how to print the string "-n" with the over 30 years-old command echo.  I tried these:  Michaels:Scripts$ echo -n Michaels:Scripts$ echo "-n" Michaels:Scripts$ echo -e "-n" Michaels:Scripts$ echo -- -n -- -n Michaels:Script$ echo -- "-n" -- -n Michaels:Script$ echo "\-n" \-n   I use Mac OS X ML. I consider this behaviour very weird and unexpected. How can I tell which cards or sets are currently legal in various formats? Diagnostics for logistic regression? For linear regression, we can check the diagnostic plots (residuals plots, Normal QQ plots, etc) to check if the assumptions of linear regression are violated.  For logistic regression, I am having trouble finding resources that explain how to diagnose the logistic regression model fit. 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