Is an ESTA enough when travelling by land from Canada to US with an RV and back to Canada with the RV? We have an Esta and are flying to Toronto and back from Calgary Canada.
I wish to travel to the US and think I am eligible to do so under the Visa Waiver Program. I found a lot of information out there but am still having some trouble understanding how this works.  What is the US Visa Waiver Program? What is an ESTA?
Xerces error: org.apache.xerces.impl.dv.dtd.DTDDVFactoryImpl According to my text book:  The Lorentz Force F on a charged particle moving in an electromagnetic field is given by:  $$F= qE + q(v\times B)$$  then it states that "it is to be pointed out that only the electric force does work, while the no work is done by the magnetic force which is simply a deflecting force."  How can this by true won't the the magnetic force have a considerable magnitude, will it now impart kinetic energy to the charge? Any explanations will help a lot. Please keep the terminology simple, thanks. What is the Linux equivalent to Windows' Program Files? Under Windows, most applications and application data are stored in a special directory known as C:\Program Files (and occasionally C:\Program Files (x86)). What is the Ubuntu/Linux equivalent to this path? Is there even one? How to convert a list of string to a list of int Move partition data with dd to the right how to auto start or enable service on next boot up or start up? in ubuntu 16.04 lts how to start or enable service on the next boot up or start up? in ubuntu 16.04 lts  without doing it manually enable or start services again everytime i start up or boot up the ubuntu 16.04 lts  thank you find examples of two series $\sum a_n$ and $\sum b_n$ both of which diverge but for which $\sum \min(a_n, b_n)$ converges Find examples of two series $\sum a_n$ and $\sum b_n$ both of which diverge but for which $\sum \min(a_n, b_n)$ converges. To make it more challenging, produce examples where $a_n$ and $b_n$ are positive and decreasing.  Edit: This problem is taken verbatim from Exercise 2.7.11 on page 68 of Abbott's Understanding analysis. QGIS 2.99 "qgis._core import error: DLL load failed" I am trying to run a standalone application. However, it does not run the program because of mentioned error in title. Here is my code:  from qgis.core import * from qgis.gui import * from qgis.utils import * from PyQt5.QtCore import * from PyQt5.QtGui import *  app = QgsApplication([], True) path = "C:/OSGeo4W64/apps/qgis-dev" app.setPrefixPath(path, True) app.initQgis()  canvas = QgsMapCanvas() title = "PyQGIS Standalone Application Example" canvas.setWindowTitle(title) canvas.setCanvasColor(Qt.white)  layer_info = 'LineString?crs=epsg:4326' layer = QgsVectorLayer(layer_info, 'MyLine', "memory") pr = layer.dataProvider() linstr = QgsFeature() wkt = "LINESTRING (1 1, 10 15, 40 35)" geom = QgsGeometry.fromWkt(wkt) linstr.setGeometry(geom) pr.addFeatures([linstr]) layer.updateExtents() QgsProject.instance().addMapLayer(layer)  canvas.setExtent(layer.extent()) canvas.setLayers([layer]) canvas.zoomToFullExtent() exitcode = app.exec_() QgsApplication.exitQgis() sys.exit(exitcode)   When I run it, I get this error:  Traceback (most recent call last):   File "C:/Users/DELL/Desktop/MyMap/myMap.py", line 1, in &lt;module&gt;     from qgis.core import *   File "C:\OSGeo4W64\apps\qgis-dev\python\qgis\__init__.py", line 72, in &lt;module&gt;     from qgis.core import QgsFeature, QgsGeometry   File "C:\OSGeo4W64\apps\qgis-dev\python\qgis\core\__init__.py", line 34, in &lt;module&gt;     from qgis._core import *   ImportError: DLL load failed: The specified module could not be found.  Python version is: 3.6 PyQt   version is: PyQt5.9 Windows 10 - 64bit  I also add many path:  %SystemRoot%\system32 %SystemRoot% %SystemRoot%\System32\Wbem %SYSTEMROOT%\System32\WindowsPowerShell\v1.0\ C:\OSGeo4W64 C:\OSGeo4W64\apps C:\OSGeo4W64\apps\gdal-dev C:\OSGeo4W64\apps\gdal-dev\bin C:\OSGeo4W64\apps\gdal-dev\pymod3\Lib\site-packages C:\OSGeo4W64\apps\grass\grass-7.2.2\bin C:\OSGeo4W64\apps\Python36 C:\OSGeo4W64\apps\Python36\Scripts C:\OSGeo4W64\apps\Python36\DLLs C:\OSGeo4W64\apps\Python36\lib\site-packages C:\OSGeo4W64\apps\qgis-dev C:\OSGeo4W64\apps\qgis-dev\bin C:\OSGeo4W64\apps\qgis-dev\lib C:\OSGeo4W64\apps\qgis-dev\python C:\OSGeo4W64\apps\qgis-dev\python\qgis C:\OSGeo4W64\apps\Qt5 C:\OSGeo4W64\apps\Qt5\lib C:\OSGeo4W64\bin C:\OSGeo4W64\apps\qgis-dev\python C:\OSGeo4W64\apps\Python36\lib C:\OSGeo4W64\include C:\Users\Mustafa Uçar\AppData\Roaming\QGIS\QGIS3\profiles\default C:\Users\Mustafa Uçar\AppData\Roaming\QGIS\QGIS3\profiles\default\QGIS   How can I fix this error? Add a non-breaking space to "reopen (1)" and its ilk When should I use "a" versus "an" in front of a word beginning with the letter h? How best to determine if an argument is not sent to the JavaScript function How to make Stack Overflow tell you about a new question with certain tag(s) the second it is asked? Formula label outside of formula How to get method parameter names? This is by far the weirdest problem I've come across,  My mac can perfectly access internet through safari web browser, but when I tried to do   ping www.google.com   through the terminal, it's requests is being timed out.  None of the other apps are able to reach internet either. Dropbox, Skype, Spotify all showing offline, except the browser itself  I'm guessing something bad with the DNS, but spend 2 hours trying to fix this with no luck Difference between int[] array and int array[] I have recently been thinking about the difference between the two ways of defining an array:   int[] array int array[]   Is there a difference? I have being using Skype for a long time on my Ubuntu 14.04 LTS. From this morning on, Skype simple cannot connect to my account anymore.  That is my Skype version:     Skype 4.3.0.37   © 2014 Skype and/or Microsoft   My Ubuntu system:  LSB Version:    core-2.0-amd64:core-2.0-noarch:core-3.0-amd64:core-3.0-noarch:core-3.1-amd64:core-3.1-noarch:core-3.2-amd64:core-3.2-noarch:core-4.0-amd64:core-4.0-noarch:core-4.1-amd64:core-4.1-noarch:cxx-3.0-amd64:cxx-3.0-noarch:cxx-3.1-amd64:cxx-3.1-noarch:cxx-3.2-amd64:cxx-3.2-noarch:cxx-4.0-amd64:cxx-4.0-noarch:cxx-4.1-amd64:cxx-4.1-noarch:desktop-3.1-amd64:desktop-3.1-noarch:desktop-3.2-amd64:desktop-3.2-noarch:desktop-4.0-amd64:desktop-4.0-noarch:desktop-4.1-amd64:desktop-4.1-noarch:graphics-2.0-amd64:graphics-2.0-noarch:graphics-3.0-amd64:graphics-3.0-noarch:graphics-3.1-amd64:graphics-3.1-noarch:graphics-3.2-amd64:graphics-3.2-noarch:graphics-4.0-amd64:graphics-4.0-noarch:graphics-4.1-amd64:graphics-4.1-noarch:languages-3.2-amd64:languages-3.2-noarch:languages-4.0-amd64:languages-4.0-noarch:languages-4.1-amd64:languages-4.1-noarch:multimedia-3.2-amd64:multimedia-3.2-noarch:multimedia-4.0-amd64:multimedia-4.0-noarch:multimedia-4.1-amd64:multimedia-4.1-noarch:printing-3.2-amd64:printing-3.2-noarch:printing-4.0-amd64:printing-4.0-noarch:printing-4.1-amd64:printing-4.1-noarch:qt4-3.1-amd64:qt4-3.1-noarch:security-4.0-amd64:security-4.0-noarch:security-4.1-amd64:security-4.1-noarch Distributor ID: Ubuntu Description:    Ubuntu 14.04.3 LTS Release:    14.04 Codename:   trusty   Any suggestion why or how to fix it?  Thanks in advance. Finishing a proof: $f$ is injective if and only if it has a left inverse I've already done a lot of searching (in particular: ) to try to prove this statement:      $f: A \to B$ is injective if and only if it has a left inverse. [I'm going to also assume $A$ and $B$ are nonempty.]   My attempt so far:     $\Leftarrow$ Suppose $f$ has a left inverse and $f(a_1) = f(a_2)$ for    $a_1, a_2 \in A$. Then there is a function $g: B \to A$ such that   $g\left(f(a_1)\right) = a_1$ and $g\left(f(a_2)\right) = a_2$. Since   $g$ is well-defined, it follows that $g\left(f(a_1)\right) =  g\left(f(a_2)\right)$, and hence $a_1 = a_2$.         $\Rightarrow$Now   suppose $f$ is injective. Then $f(a_1) = f(a_2) \implies a_1 = a_2$.   Furthermore, since $A$ is nonempty, there is an element $x \in A$.   Define $g: B \to A$ by  \begin{equation*} g(b) = \begin{cases}  f^{-1}(b), &amp; b \in \text{Im}(f) \\ x, &amp; b \in A \setminus  \text{Im}(f)\text{.} \end{cases} \end{equation*} This mapping is   well-defined, since if $f$ is injective, $f^{-1}(b)$ is unique.   My main question: does this imply that if $f: A \to B$ is injective that any mapping $g: \text{Im}(f) \to A$ is bijective? Otherwise, I don't understand why we can even use $f^{-1}$ on an element. [Please read the link above for more details - in proof 1.] Extracting extension from filename in Python iOS 11-13 - unable to paste into comments I’m able to copy-paste from any application to any application. I’m able to copy from a comment or from a question or from an answer. I’m able to paste to a question or answer. However, if I attempt to paste into a comment, nothing happens. I am able to subsequently paste into another application, confirming that the text is actually on the clipboard.   However, copying SE links will paste them successfully, but all other text - including other links - still fails. This leads me to believe that it’s a SE issue and not an Apple one.   This has been an issue since 11.0 beta and persists through iOS 13 release.