(defun prime (a)  ;;判斷質數的function
    (setq x (round (sqrt a)))  ;;令x是小於根號a的數字
    (defun p (i)
        (if (= (mod a i) 0)  ;;第一次呼叫是從最接近根號a的數字開始 餘數=0就不是質數
            ()
            (if (= (setq i (- i 1)) 1)  ;;餘數!=0就把i-1 若i!=1表示還沒判斷完
                (print a)
            (p i)  ;;沒判斷完就繼續呼叫
            )
        )
    )
    (if (= a 2) 
         (print a)  ;;如果a=2必為質數所以印出
    (p x)  ;;其他放入p這個function判斷
    )
)
 
 
(do ;;((a 2(+ a 1)))      
        ((= a 257)) ;;由1~1000一個一個放進function判斷
   (prime a)
)

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KHByaW1lIDUp

(prime 5)