Update index.md

Author: arjendeetman

index.md

@@ -56,7 +56,7 @@ The plugin is an open source project that was initiated by the chair of [Experim
 
 Robot Components uses the ABB PC SDK for real-time connection to ABB Robots, you can find the .dll used in this project here: [ABB developercenter](http://developercenter.robotstudio.com/landing). 
 
-Robot Components uses the OPW kinematics solver as described in the paper '_An Analytical Solution of the Inverse Kinematics Problem of Industrial Serial Manipulators with an Ortho-parallel Basis and a Spherical Wrist_' by Mathias Brandstötter, Arthur Angerer, and Michael Hofbaur.
+Robot Components uses the OPW kinematics solver as described in the paper ['_An Analytical Solution of the Inverse Kinematics Problem of Industrial Serial Manipulators with an Ortho-parallel Basis and a Spherical Wrist_'](https://www.researchgate.net/publication/264212870_An_Analytical_Solution_of_the_Inverse_Kinematics_Problem_of_Industrial_Serial_Manipulators_with_an_Ortho-parallel_Basis_and_a_Spherical_Wrist) by Mathias Brandstötter, Arthur Angerer, and Michael Hofbaur.
 
 We would like to acknowledge [Jose Luis Garcia del Castillo](https://github.com/garciadelcastillo) and [Vicente Soler](https://github.com/visose) for making their Grasshopper plugins [RobotExMachina](https://github.com/RobotExMachina) and [Robots](https://github.com/visose/Robots) available. Even though our approach is different it was helpful for us to see how you implemented certain functionalities and approached certain issues.