diff --git a/src/reihen.c b/src/reihen.c
index b58b27e..d194950 100644
--- a/src/reihen.c
+++ b/src/reihen.c
@@ -80,4 +80,18 @@ double tanD(double degrees){
     }
     return sinD(degrees)/cosD(degrees);
 
+}
+
+double atanD(double degrees){
+    if (degrees>=360.0){
+            degrees = squashDegreesTo360(degrees);
+    }
+
+    double sum = 0.0;
+    double radian = radians(degrees);
+    for (int i = 0; i <= 9; i++){
+        sum += powerD((double) i, -1.0) * (powerD(2.0*i+1, radian) / 2.0*i+1);
+    }
+
+    return sum;
 }
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