(ql:quickload '(lparallel))

(setf lparallel:*kernel* (lparallel:make-kernel 4)) ;; Configure for your system.

(defun factor (n &optional (acc '()))
  (when (> n 1)
    (loop with max-d = (isqrt n)
       for d = 2 then (if (evenp d) (1+ d) (+ d 2)) do
         (cond ((> d max-d) (return (cons (list n 1) acc)))
               ((zerop (rem n d))
                (return (factor (truncate n d)
                                (if (eq d (caar acc))
                                    (cons
                                     (list (caar acc) (1+ (cadar acc)))
                                     (cdr acc))
                                    (cons (list d 1) acc)))))))))

(defun max-minimum-factor (numbers)
  (lparallel:pmap-reduce
   (lambda (n) (cons n (apply #'min (mapcar #'car (factor n)))))
   (lambda (a b) (if (> (cdr a) (cdr b)) a b))
   numbers))

(defun print-max-factor (pair)
  (format t "~a has the largest minimum factor ~a~%" (car pair) (cdr pair)))