diff --git a/README.md b/README.md
index 1225d7f..f4cccbe 100644
--- a/README.md
+++ b/README.md
@@ -24,3 +24,4 @@ Extented Kalman filter for attitude estimation using imu and magnetic filed.
     
 # reference: 
 https://pixhawk.org/_media/firmware/apps/attitude_estimator_ekf/ekf_excerptmasterthesis.pdf
+http://web.cs.iastate.edu/~cs577/handouts/quaternion.pdf
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