lispdoc  results for isqrt 
(isqrt n)  Function: Return the root of the nearest integer less than n which is a perfect square.

Mentioned in: CLtL2  12.5.1. Exponential and Logarithmic Functions HyperSpec  Function SQRT, ISQRT  
(sqrt number)  Function: Return the square root of NUMBER.

Example:(defun mag (x y z) (sqrt (+ (sq x) (sq y) (sq z))))  Mentioned in: CLtL2  12.1. Precision, Contagion, and Coercion CLtL2  12.5.1. Exponential and Logarithmic Functions CLtL2  12.5.3. Branch Cuts, Principal Values, and Boundary Conditions in the Complex Plane HyperSpec  Function SQRT, ISQRT On Lisp  Operations on Lists 
By Bill Moorier 