lispdoc  results for denominator 
(denominator number)  Function: Return the denominator of NUMBER, which must be rational.

Example:(defun ratdenominator (rat) "The denominator of a rational expression." (typecase rat (cons (cdr rat)) (number (denominator rat)) (t 1)))  Mentioned in: CLtL2  12.6. Type Conversions and Component Extractions on Numbers CLtL2  22.1.6. What the Print Function Produces HyperSpec  Function NUMERATOR, DENOMINATOR Successful Lisp  assert 
By Bill Moorier 