1 ! Copyright (c) 2009 Guillaume Nargeot.
2 ! See http://factorcode.org/license.txt for BSD license.
3 USING: kernel math lists lists.lazy project-euler.common sequences ;
6 ! http://projecteuler.net/index.php?section=problems&id=065
11 ! The square root of 2 can be written as an infinite continued fraction.
14 ! √2 = 1 + -------------------------
16 ! 2 + ---------------------
18 ! 2 + -----------------
23 ! The infinite continued fraction can be written, √2 = [1;(2)], (2) indicates
24 ! that 2 repeats ad infinitum. In a similar way, √23 = [4;(1,3,1,8)].
26 ! It turns out that the sequence of partial values of continued fractions for
27 ! square roots provide the best rational approximations. Let us consider the
31 ! 1 + - = - ; 1 + ----- = - ; 1 + --------- = -- ; 1 + ------------- = --
33 ! 2 + - 2 + ----- 2 + ---------
40 ! Hence the sequence of the first ten convergents for √2 are:
41 ! 1, 3/2, 7/5, 17/12, 41/29, 99/70, 239/169, 577/408, 1393/985, 3363/2378, ...
43 ! What is most surprising is that the important mathematical constant,
44 ! e = [2; 1,2,1, 1,4,1, 1,6,1 , ... , 1,2k,1, ...].
46 ! The first ten terms in the sequence of convergents for e are:
47 ! 2, 3, 8/3, 11/4, 19/7, 87/32, 106/39, 193/71, 1264/465, 1457/536, ...
49 ! The sum of digits in the numerator of the 10th convergent is 1+4+5+7=17.
51 ! Find the sum of digits in the numerator of the 100th convergent of the
52 ! continued fraction for e.
62 dup 3 mod zero? [ 3 / 2 * ] [ drop 1 ] if
66 1 - (e-frac) ltake list>array reverse 0
67 [ + recip ] reduce 2 + ;
71 : euler065 ( -- answer )
72 100 e-frac numerator number>digits sum ;
74 ! [ euler065 ] 100 ave-time
75 ! 4 ms ave run time - 0.33 SD (100 trials)