Q:

Could not resolve com.android.support:appcompat-v7:26.1.0

After upgrading to androidstudio 3.0 gradle stopped working:
Unable to resolve dependency for ':app@debug/compileClasspath': Could not resolve com.android.support:appcompat-v7:26.1.0. 
Could not resolve com.android.support:appcompat-v7:26.1.0. 
Required by:
    project :app

Could not resolve com.android.support:appcompat-v7:26.1.0. 
  Could not get resource 'https://dl.google.com/dl/android/maven2/com/android/support/appcompat-v7/26.1.0/appcompat-v7-26.1.0.pom'. 
  Could not GET 'https://dl.google.com/dl/android/maven2/com/android/support/appcompat-v7/26.1.0/appcompat-v7-26.1.0.pom'. 
  Connect to dl.google.com:443 [dl.google.com/172.217.22.110] failed: Connection timed out: connect
  Connection timed out: connect

I have checked the proxy and it works. 
This is a new project, nothing was changed from what the android studio wizard generates. 
Thanks for your help.  

A:

Try adding Google maven repository into app's build.gradle file: 
repositories {
    maven { url "https://maven.google.com" }
}

