My answers of projecteuler.net problems.

problem-1.lisp

#|
    Multiples of 3 and 5
    (https://projecteular.net/problem=1)

    1,000以下の自然数で3か5の倍数である数の和を求める
|#

(defun sum-of-multipes-of-3-or-5 (m)
        (loop for n
              below m
              when (or (eq (mod n 3) 0)
                       (eq (mod n 5) 0))
              sum n))

(defun sum-of-multiple (x m)
  (let ((n (floor (float (/ (1- m) x)))))
    (* (/ (* n (1+ n)) 2) x)))

(defun solve-1 ()
  (sum-of-multipes-of-3-or-5 1000))

(defun solve-2 ()
  (- (+ (sum-of-multiple 3 1000) (sum-of-multiple 5 1000)) (sum-of-multiple 15 1000)))

problem-2.lisp

#|
    Even Fibonacci numbers
    https://projecteuler.net/problem=2

    4,000,000未満のフィボナッチ数列の偶数項の和を求める
|#

(defun fib (lst)
  (cons (+ (car lst) (cadr lst)) lst))

(defun fib-to-n (lst n)
  (if (> (+ (car lst) (cadr lst)) n)
      lst
      (fib-to-n (fib lst) n)))

(defun sum-of-even (lst)
  (loop for i
        in lst
        when (evenp i)
        sum i))

(defun solve-1 ()
  (sum-of-even (fib-to-n (list 3 2 1) 4000000)))

problem-3.lisp

#|
    Largest prime factor
    https://projecteuler.net/problem=3

    600,851,475,143の最大素因数を求める
|#

(defun prime-list (num)
  (labels ((f (n lst y)
           (if (eq n 1) lst
             (if (eq (mod n y) 0) (cons y (f (/ n y) lst y))
               (f n lst (1+ y))))))
    (f num () 2)))

(defun solve-1 ()
  (car (last (prime-list 600851475143))))

problem-4.lisp

#|
    Largest palindrome product
    https://projecteuler.net/problem=4

    ３桁×３桁からなる回文数のうち最大の数を求める
|#

(defun is-palindrome (num)
  (let ((lst (concatenate 'list (write-to-string num))))
    (labels ((f (n)
             (if (>= n (list-length lst)) n
               (if (eq (nth n lst) (nth (- (1- (list-length lst)) n) lst)) (f (1+ n))
                 nil
             ))))
      (f 0))))

(defun make-palindrome-list (i j)
  (if (>= 100 i) nil
    (if (>= 100 j) (make-palindrome-list (1- i) 999)
      (if (is-palindrome (* i j)) (cons (* i j) (make-palindrome-list i (1- j)))
        (make-palindrome-list i (1- j))))))

(defun solve-1 ()
  (apply #'max (make-palindrome-list 999 999)))