1 ! Copyright (c) 2008 Aaron Schaefer.
2 ! See https://factorcode.org/license.txt for BSD license.
3 USING: kernel math project-euler.common sequences ;
6 ! https://projecteuler.net/problem=71
11 ! Consider the fraction, n/d, where n and d are positive
12 ! integers. If n<d and HCF(n,d) = 1, it is called a reduced
15 ! If we list the set of reduced proper fractions for d <= 8 in
16 ! ascending order of size, we get:
18 ! 1/8, 1/7, 1/6, 1/5, 1/4, 2/7, 1/3, 3/8, 2/5, 3/7, 1/2, 4/7, 3/5, 5/8,
19 ! 2/3, 5/7, 3/4, 4/5, 5/6, 6/7, 7/8
21 ! It can be seen that 2/5 is the fraction immediately to the
24 ! By listing the set of reduced proper fractions for d <=
25 ! 1,000,000 in ascending order of size, find the numerator of
26 ! the fraction immediately to the left of 3/7.
32 ! Use the properties of a Farey sequence by setting an upper
33 ! bound of 3/7 and then taking the mediant of that fraction and
34 ! the one to its immediate left repeatedly until the denominator
35 ! is as close to 1000000 as possible without going over.
37 : euler071 ( -- answer )
38 2/5 [ dup denominator 1000000 <= ] [ 3/7 mediant dup ] produce
39 nip penultimate numerator ;
41 ! [ euler071 ] 100 ave-time
42 ! 155 ms ave run time - 6.95 SD (100 trials)