How to disable beep sound in Linux CentOS 7 command line?
How to disable system beep for non-privileged user
When is "seems to be" used instead of "seems"? The baby seems happy/comfortable. The baby seems to be happy/comfortable.   Are these both correct? If both are correct, what are the differences between them? Find the limit of $\sum\limits_{k=1}^n\left(\sqrt{1+\frac{k}{n^2}}-1\right)$ I have written code like below and all works fine, but while submitting the form I get the error below. I am trying to filter the second dropdown based on the first drop down selection.     An illegal choice has been detected. Please contact the site administrator.   How do I overcome that error?       function dynamic_location_dropdown_form_alter(&amp;$form, $form_state, $form_id) {   if($form_id == 'product_node_form') {    $location_options = array();    if(isset($form['field_destination']['und']['#default_value'][0])) {     $destination = $form['field_destination']['und']['#default_value'][0];   }   else {     $destination = 0;   }    $location_options = dynamic_location_dropdown_locations($destination);    $form['field_destination']['und']['#ajax'] = array(     'event' =&gt; 'change',     'wrapper' =&gt; 'squadron-wrapper',     'callback' =&gt; 'dynamic_location_dropdown_ajax_callback',     'method' =&gt; 'replace',   );   $form['field_product_location']['#validated'] = true;   $form['field_product_location']['und']['#prefix'] = '&lt;div id="squadron-wrapper"&gt;';   $form['field_product_location']['und']['#suffix'] = '&lt;/div&gt;';   $form['field_product_location']['und']['#options'] = $location_options;   } }  function dynamic_location_dropdown_ajax_callback($form, $form_state) {   $country_id = $form['field_destination']['und']['#value'];   $form['field_product_location']['#validated'] = true;   $form['field_product_location']['und']['#options'] =  dynamic_location_dropdown_locations($country_id);    return $form['field_product_location']; }  function dynamic_location_dropdown_locations($destination_id) {   $nodes = array();   $nodes[''] = '- None -';   if($destination_id != '') {     $select = db_query("       SELECT node.title AS node_title, node.nid AS nid, node.created AS node_created       FROM {node} node       LEFT JOIN {field_data_field_location_country} field_data_field_location_country        ON node.nid = field_data_field_location_country.entity_id        AND (field_data_field_location_country.entity_type = 'node'        AND field_data_field_location_country.deleted = '0')       WHERE (( (node.status = '1')        AND (node.type IN  ('location'))        AND (field_data_field_location_country.field_location_country_nid = $destination_id)))       ORDER BY node_title ASC     ");     $nodes[''] = '- None -';     foreach ($select as $node) {       $nodes[$node-&gt;nid] = $node-&gt;node_title;     }   }    return $nodes; } In what cases should I use a comma before "or"? Idiom/Phrase for "results without work/effort" I'd be greatly appreciative of a cleverly devised idiom/phrase that depicts the following concept:     Results without work/effort.   ...and/or...     My Work/Effort; Your Result.   Any idioms/phrases you can recount or conceive would be of use.    Contextual Edit:  The idiom/phrase should be in the context of service offering/hiring or task delegation, not free-loading as most answers thus far have understandably assumed. E.g.     [You hire me for] My Work/Effort; [My work/effort produces] Your   Result. Ubuntu system is broken after accidentally uninstalling many packages I am currently visiting the USA from the UK (I am a UK citizen) and am intending on staying here for the full 90 days under the Visa Waiver Program.     How long would I have to stay out of the US at the end of this period before returning again?    Would I have to return to the UK, or could I stay with friends in another country (such as Canada) for a while? What makes a Feynman diagram real or virtual? Simple question: as the title says, what makes a real Feynman diagram real, and what makes a virtual diagram virtual? Or in other words, how do I tell whether any given diagram is real or virtual? I've never gotten a really satisfying explanation of this. I would imagine it has something to do with virtual particles, but all internal propagators are virtual particles and I know for a fact that having internal lines doesn't make a diagram virtual. Relation between AB and BA Let $A$ and B be $n\times n$ matrices over a field $F$. Then prove or disprove  $(i)\, AB$ and $BA$ have same characteristic values.  $(ii)\, AB$ and $BA $ have same characteristic polynomial.  $(iii)\, AB$ and $BA $ have same minimal polynomial.  Proof of $(i)$   Let $c$ be an eigen value of AB and v be corresponding eigen vector. Then    AB(v)=cv,    B(ABv)=B(cv),    BA(Bv)=c(Bv),   this implies $Bv$ is an eigenvector of $BA$ corr. to eigen value $c$.  But this statement holds if $Bv$ is nonzero.  What if $Bv=0$?  $(ii)$ If $A$ is invertible then ($A$ inverse)$(AB)A= BA$, i.e. $AB $ and $BA$ are similar and hence have same charateristic polynomial.   Similar proof when $B$ is invertible.  But what if both $A$ and $B$ are not invertible?  I have no idea about $(iii)$.  Please help me to complete these proofs. Selecting text in profile activity comments causes unexpected clipping This is a very minor bug I know, but in anyone's profile page, on the Activity or Responses tab, text highlighting is messed up for the Comment and All subcategories. This only seems to happen only in Chrome. I am on Version 31.0.1650.63 (Win 7). It is visibly worse on multiple lines of text.     Steps to repro:   Go to any profile on any Stack Exchange site, and click on Activity tab. Go to All or Comment subcategories. Highlight text.   As a side effect, you can even temporarily dismember the head of all those poor characters. To do this, start from any line, drag mouse left or right, drag up and then back down.    I've bumped into bugs before that turned out to be Chrome problems, so maybe this is just Chrome doing its magic. Don't count non-printing characters, HTML comments or other content not displayed toward minimum character requirements Using dpkg to install upgrade and dist-upgrade packages Package denyhosts in Ubuntu Trusty Tahr is deleted: temporary or forever? While doing a test-upgrade of our Ubuntu server to 14.04, I found that the package DenyHosts is no longer available. Installing it gives following error:  apt-get install denyhosts Reading package lists... Done Building dependency tree        Reading state information... Done E: Unable to locate package denyhosts   Apparently it has been deleted, according to .  Will Denyhosts be available in the final release of Ubuntu 14.04? Definitions and usage of Covariant, Form-invariant & Invariant? Just wondering about the definitions and usage of these three terms.  To my understanding so far, "covariant" and "form-invariant" are used when referring to physical laws, and these words are synonyms?  "Invariant" on the other hand refers to physical quantities?  Would you ever use "invariant" when talking about a law? I ask as I'm slightly confused over a sentence in my undergrad modern physics textbook:     "In general, Newton's laws must be replaced by Einstein's relativistic laws...which hold for all speeds and are invariant, as are all physical laws, under the Lorentz transformations." [emphasis added]      ~ Serway, Moses &amp; Moyer. Modern Physics, 3rd ed.   Did they just use the wrong word? What could cause touch to fail with this error message?  touch: cannot touch `foo': No such file or directory   Note that an error due to incorrect permissions looks different:  touch: cannot touch `foo': Permission denied Bypass GPG signature checks only for a single repository Creating Inky-looking characters in Blender The title is probably very confusing, but here's the situation:  I'm animating a flat grid curve by shrink-wrapping it onto a moving plane in order to get a uniform wave animation, it looks like this:    Before I render it, i want a bevel. What I did then was simply adding a bevel to the curve, making the curve look like this:    Now, when activating the shrinkwrap modifier, I expexted that only the curve would be shrinkwrapped, but alas, the bevel did the same:    As seen by the blue spots, the curve has been completely flattened along with the bevel.  How can I avoid this? I tried using a wave modifier instead of shrinkwrap, but this modifier doesn't seem to make perfectly round waves like this:    So, any thoughs on how to solve this?  Thanks for reading! I'm reading .     The graph of $f: A \rightarrow B$ is the subset of $A × B$ consisting   of all ordered pairs $(a, b)$ with $f (a) = b$. If $A$ happens to be   $B^n$ for some $n ∈ N$, then we say that $f$ is a function on $B$ and   $n$ is the arity of $f$.   I don't understand the notation for the exponent of a set, I'm aware of what  is, it's the number of arguments or operands the function takes. I'm just not so sure if I can assume that a set with an exponent such as in $B^n$ means that a function has two operands.  I know the question may be too naive and perhapes the answer is the one I suggested and it's right in front of my face, I'm just searching for a confirmation or the negation of it. Thanks. I would like to know if there is any man page documenting the construction of the most basic script commands like if/then/else, while, for each, and all the relative switches, like -eq, -e, -ge, and so on.