lispdoc  results for atan 
(atan y &optional (x nil xp))  Function: Return the arc tangent of Y if X is omitted or Y/X if X is supplied.

Mentioned in: CLtL2  12.5.1. Exponential and Logarithmic Functions CLtL2  12.5.2. Trigonometric and Related Functions CLtL2  12.5.3. Branch Cuts, Principal Values, and Boundary Conditions in the Complex Plane HyperSpec  Function ASIN, ACOS, ATAN PCL  higher math  
(tan number)  Function: Return the tangent of NUMBER.

Mentioned in: CLtL2  12.5.2. Trigonometric and Related Functions CLtL2  12.5.3. Branch Cuts, Principal Values, and Boundary Conditions in the Complex Plane HyperSpec  Function SIN, COS, TAN PCL  higher math  
By Bill Moorier 