initial commit

1 files changed, 41 insertions, 0 deletions

diff --git a/euler12.scm b/euler12.scm
new file mode 100644
index 0000000..e0d726f
--- /dev/null
+++ b/euler12.scm
@@ -0,0 +1,41 @@
+(define (square x) (* x x))
+(define (power x n)
+ (if (= n 0)
+  1
+  (* x (power x (- n 1)))))
+
+(define (first-divisor x)
+ (define (try-iter d)
+  (cond ((= 0 (remainder x d)) d)
+   		((> (square d) x) x)
+		(else (try-iter (+ d 1)))))
+ (try-iter 2))
+
+(define (divexp x d)
+ (if (= (remainder x d) 0)
+	(+ (divexp (/ x d) d) 1)
+	0))
+	
+(define (ndiv n)
+ (define (pair d)
+  (cons (divexp n d) (/ n (expt d (divexp n d)))))
+ (if (= n 1) 1
+  (let ((fd (first-divisor n)))
+   (* (+ 1 (car (pair fd)))
+     (ndiv (cdr (pair fd)))))))
+
+(define (kth-factor-triangle-over-n n)
+ (define (n-fac-tri n)
+  (if (even? n)
+   (* (ndiv (/ n 2)) (ndiv (+ n 1)))
+   (* (ndiv n) (ndiv (/ (+ n 1) 2)))))
+ (define (try-iter i)
+  (if (> (n-fac-tri i) n)
+   i
+   (try-iter (+ i 1))))
+ (try-iter 1))
+
+(display 
+ ((lambda (x) (/ (* x (+ x 1)) 2))
+  (kth-factor-triangle-over-n 500)))
+(newline)