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How to resolve a Maven dependency of type pom?

I am trying to import a library in Maven which is Kotlin Multiplatform.
This is it's Github repo (does not actually matter much of course)
Point is, it says it can be imported in with this dependency for Gradle:
dependencies {
implementation("com.github.ajalt.colormath:colormath:2.0.0")
}
Obviously, just converting it to Maven does not work:
<dependencies>
...
<dependency>
<groupId>com.github.ajalt.colormath</groupId>
<artifactId>colormath</artifactId>
<version>2.0.0</version>
<type>jar</type>
</dependency>
</dependencies>
So I added it's pom to the project as this answer explains it:
<dependencyManagement>
<dependencies>
<dependency>
<groupId>com.github.ajalt.colormath</groupId>
<artifactId>colormath</artifactId>
<version>2.0.0</version>
<type>pom</type>
<scope>import</scope>
</dependency>
</dependencies>
</dependencyManagement>
This dependency is resolved (it can be only resolved as pom type. Then I add it as dependency:
<dependencies>
...
<dependency>
<groupId>com.github.ajalt.colormath</groupId>
<artifactId>colormath</artifactId>
<version>2.0.0</version>
<type>jar</type>
</dependency>
</dependencies>
It still cannot find it.
I also added the repo1, because I did not see where Maven was looking for this artifact:
<repositories>
<repository>
<id>central</id>
<name>Central Repository</name>
<url>https://repo1.maven.org/maven2</url>
</repository>
</repositories>
But still no success. I don't understand why it's not working.
The code can be found in the repo1, but Maven does not resolve it.
This must be a very simple thing I am not understanding here, please help.

The link to the Maven Central from GitHub points to a bare POM artifact, without any dependencies and/or attached JARs.
It looks like Kotlin MP uploads JVM-specific artifacts with a different artifactId - colormath-jvm in this case. Please check corresponding directory in the Maven Central.
I suggest using following dependency declaration in the POM:
<groupId>com.github.ajalt.colormath</groupId>
<artifactId>colormath-jvm</artifactId>
<version>2.0.0</version>

Maven: NoClassDefFoundError of system scoped dependency

I am developing an Android project with Maven. I have a third-party jar file that I have included in a lib folder on my project root directory:
<dependency>
<groupId>com.parse</groupId>
<artifactId>parse</artifactId>
<version>1.9.2</version>
<scope>system</scope>
<systemPath>${project.basedir}\lib\Parse-1.9.2.jar</systemPath>
</dependency>
But when I install the apk in my phone, I get NoClassDefFoundError. Obviously, that class exists inside the jar.
How can I do?
Thanks.

I would suggest using one of your directories as a repository, add your jar in it, and load it the proper Maven way.
You can do this adding this to your pom.xml and putting your jar in /lib:
<repositories>
<repository>
<id>mylibid</id>
<url>file://${project.basedir}/lib</url>
</repository>
</repositories>
...
<dependencies>
...
<dependency>
<groupId>com.parse</groupId>
<artifactId>parse</artifactId>
<version>1.9.2</version>
</dependency>
...
</dependencies>

Add my two cents here, you can install the third-party into your local repository firstly and try add dependency directly.
Take your scenario as an example. Change to the folder where contains the third jar and type:
mvn install:install-file -DgroupId=com.parse -DartifactId=parse -Dversion=1.9.2 -Dpackaging=jar -Dfile=Parse-1.9.2.jar
Modify your pom.xml to add the dependency.
<dependencies>
...
<dependency>
<groupId>com.parse</groupId>
<artifactId>parse</artifactId>
<version>1.9.2</version>
</dependency>
...
</dependencies>

What is the difference between the GeoIP2 MaxMind pom and my local one?

I'm following this guide:
https://github.com/maxmind/GeoIP2-java
It says:
We recommend installing this package with Maven. To do this, add the dependency to your pom.xml:
<dependency>
<groupId>com.maxmind.geoip2</groupId>
<artifactId>geoip2</artifactId>
<version>2.2.0</version>
</dependency>
There is also pom.xml file in the Git repository of GeoIP2 which is much longer - what is the difference between them?

Cited from the official homepage:
Apache Maven is a software project management and comprehension tool. Based on the concept of a project object model (POM), Maven can manage a project's build, reporting and documentation from a central piece of information.
Think of the pom.xml as the heart of Maven. In the file you can specify dependencies (most typically jar files), and other information, such as how the project should be built. Without digging to deep into this, one of Maven's strengths is that it manages the dependencies of projects.
To answer your concrete question, GeoIP2 manages its dependencies using Maven. This section of its pom.xml defines them:
<dependencies>
<dependency>
<groupId>com.maxmind.db</groupId>
<artifactId>maxmind-db</artifactId>
<version>1.0.0</version>
</dependency>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>4.12</version>
<scope>test</scope>
</dependency>
<dependency>
<groupId>com.google.http-client</groupId>
<artifactId>google-http-client</artifactId>
<version>1.20.0</version>
</dependency>
<dependency>
<groupId>com.fasterxml.jackson.core</groupId>
<artifactId>jackson-databind</artifactId>
<version>2.5.3</version>
</dependency>
</dependencies>
By using Maven in your own project, you will only need to add the one dependency to GeoIP2. Maven will then search for the dependency in a repository, typically the Maven Central Repository if Maven isn't configured to use another. It will also automatically download all other needed dependencies (transitive dependencies), in this case it would be the dependencies listed above, plus any other dependencies those in turn depend on, and so on.
So, a short recap: Without a dependency management tool like Maven, you would need to manually make sure you have all the correct dependencies on the classpath. Maven fixes this for you.

How to get sources in my project

I'm trying to get sources added to my project. I manually added the below repo and included an atmosphere snapshot.
<repositories>
<!-- Added to get the Atmosphere 1.1.0-SNAPSHOT, can be removed when 1.1.0 is released -->
<repository>
<id>oss.sonatype.org-snapshot</id>
<url>http://oss.sonatype.org/content/repositories/snapshots</url>
<releases><enabled>false</enabled></releases>
<snapshots><enabled>true</enabled></snapshots>
</repository>
</repositories>
<dependencies>
<dependency>
<groupId>org.atmosphere</groupId>
<artifactId>atmosphere-runtime</artifactId>
<version>1.1.0-SNAPSHOT</version>
<exclusions>
<exclusion>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-api</artifactId>
</exclusion>
</exclusions>
</dependency>
</dependencies>
I went and downloaded the .jar from here but when I add it to the "libraries" section in my Project Structure it doesn't add it as a source, but adds it under Classes.
In debugging when viewing the interface of a class, I've used the quick "Attach sources..." option at the top of the editor and navigated to the .jar with no avail.

Maven doesn't find the sources, because there are no sources of that library in that Maven repository (which i think is quite common for snapshots of a library). I downloaded one of the jars, and they contain only class files, so i'm wondering, how you can see the sources even if you attach the jar manually.

Can't find the "org.eclipse.persistence" Maven dependency

I installed Eclipse Helios with the m2eclipse maven plugin.
I want to create an application using JPA. So, what I do is: New > Maven Project then I select the maven default archetype.
The problem is that I want to add the "org.eclipse.persistence" dependency that I can't find.
Where is it? Can we add it manually? Should I update a sort of "repository"?
Then, is it the right archetype that I'm using?

EclipseLink is not available in Maven central repository, you need to add its repository manually. For example, to use the "full" version of EclipseLink 2.0 (you didn't mention the artifact you're looking for):
<dependencies>
<dependency>
<groupId>org.eclipse.persistence</groupId>
<artifactId>eclipselink</artifactId>
<version>2.0.0</version>
<scope>compile</scope>
...
</dependency>
<dependencies>
...
<repositories>
<repository>
<id>EclipseLink Repo</id>
<url>http://www.eclipse.org/downloads/download.php?r=1&nf=1&file=/rt/eclipselink/maven.repo</url>
</repository>
...
</repositories>
This is documented in the EclipseLink/Maven page.
Regarding the archetype you're using, it's impossible to answer without more details on the kind of project you want to create. And anyway, you can always modify the POM after the facts.

<dependency>
<groupId>org.eclipse.persistence</groupId>
<artifactId>javax.persistence</artifactId>
<version>2.0.0</version>
<scope>compile</scope>
</dependency>
...
<repositories>
<repository>
<url>http://repo.maven.apache.org/maven2</url>
</repository>
</repositories>

You can check below link. I found eclipse JARs on this link.
However, no idea about how to add it on Nexus.
http://dev.nightlabs.org/maven-repository/repo/