Project Euler

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# [[Problem 3]]

## Largest Prime Factor

The prime factors of $13195$ are $5, 7, 13$ and $29$.

What is the largest prime factor of the number $600851475143$?

## 最大の素因数

13195 の素因数は 5, 7, 13, 29 である.

600851475143 の素因数のうち最大のものを求めよ.

```scheme

#!r6rs

#!chezscheme

(import (chezscheme))

(define (prime-factor num)

  (let ([check-max (isqrt num)])

    (let loop ([current 2] [rest num] [result '()])

      (cond

        [(= rest 1) result]

        [(< check-max current) (cons rest result)]

        [(zero? (mod rest current))

         (loop current (div rest current) (cons current result))]

        [else (loop (add1 current) rest result)]))))

(define answer-3

  (apply max (prime-factor 600851475143)))

(printf "3: ~D~%" answer-3)

```