commit bdfff4a9a2594575d2d9e11d5edbd4207b7e7358
Author: Oliver Payne <ompayne@gmail.com>
Date:   Mon May 10 21:57:13 2021 +0100

    Initial commit

diff --git a/1_10.sch b/1_10.sch
new file mode 100644
index 0000000..6c08dec
--- /dev/null
+++ b/1_10.sch
@@ -0,0 +1,21 @@
+(define (A x y)
+  (cond ((= y 0) 0)
+	((= x 0) (+ 2 y))
+	((= y 1) 2)
+	(else (A (- x 1)
+		 (A x (- y 1))))))
+
+; 2+n
+(define (f n)
+  (A 0 n))
+
+; 2*n
+(define (g n)
+  (A 1 n))
+
+; 2^n
+(define (h n)
+  (A 2 n))
+
+(define (k n)
+  (* 5 n n))
diff --git a/1_11.sch b/1_11.sch
new file mode 100644
index 0000000..273bcf1
--- /dev/null
+++ b/1_11.sch
@@ -0,0 +1,23 @@
+; f(n) = n, n < 3, f(n) = f(n-1) + 2f(n-2) + 3f(n-3) n >= 3
+
+(define (f1 n)
+  (if (< n 3)
+      n
+      (+ (f1 (- n 1))
+	 (* 2 (f1 (- n 2)))
+	 (* 3 (f1 (- n 3))))))
+
+(define (f2 n)
+  (if (< n 3)
+      n
+      (f-iter 2 1 0 n)))
+
+; a = f(2), b = f(1), c = f(0)
+; a <- a + 2b + 3c, b <- a, c <- b
+(define (f-iter a b c count)
+  (if (< count 3)
+      a
+      (f-iter (+ a (* 2 b) (* 3 c))
+	      a
+	      b
+	      (- count 1))))
diff --git a/1_12.sch b/1_12.sch
new file mode 100644
index 0000000..c0d6524
--- /dev/null
+++ b/1_12.sch
@@ -0,0 +1,6 @@
+(define (pascal row col)
+  (cond
+   ((= col 1) 1)
+   ((or (= col 1) (= col row)) 1)
+   (else (+ (pascal (- row 1) (- col 1))
+      (pascal (- row 1) col)))))
diff --git a/1_16.sch b/1_16.sch
new file mode 100644
index 0000000..9a51660
--- /dev/null
+++ b/1_16.sch
@@ -0,0 +1,11 @@
+(define (expt1 b n)
+  (expt-iter b n 1))
+
+(define (expt-iter b n a)
+  (cond ((= n 0) a)
+	((odd? n)
+	       (expt-iter b (- n 1) (* a b)))
+	(else (expt-iter (square b) (/ n 2) a))))
+
+(define (square x) (* x x))
+
diff --git a/1_17.sch b/1_17.sch
new file mode 100644
index 0000000..92ee256
--- /dev/null
+++ b/1_17.sch
@@ -0,0 +1,17 @@
+(define (mult-rec a b)
+  (cond ((= b 0) 0)
+	((even? b) (mult-rec (double a) (halve b)))
+	(else (+ a (mult-rec a (- b 1))))))
+
+(define (mult-iter a b n)
+  (cond ((= b 0) n)
+	((even? b) (mult-iter (double a) (halve b) n))
+	(else (mult-iter a (- b 1) (+ a n)))))
+
+(define (mult a b)
+  (mult-iter a b 0))
+
+(define (double a) (+ a a))
+
+(define (halve a) (/ a 2))
+
diff --git a/1_21.sch b/1_21.sch
new file mode 100644
index 0000000..bfa3fc3
--- /dev/null
+++ b/1_21.sch
@@ -0,0 +1,125 @@
+(import chicken.time)  ; for current-milliseconds
+(import chicken.random)  ; for pseudo-random-integer
+
+(define (smallest-divisor n)
+  (find-divisor n 2))
+
+(define (find-divisor n test-divisor)
+  (cond ((> (square test-divisor) n) n)
+	((divides? test-divisor n) test-divisor)
+	(else (find-divisor n (+ test-divisor 1)))))
+
+(define (divides? a b)
+  (= (remainder b a) 0))
+
+;(define (prime? n)
+  ;(= n (smallest-divisor n)))
+
+(define (square x) (* x x))
+
+
+(define (timed-prime-test n)
+  ;(newline)
+  ;(display n)
+  (start-prime-test n (current-milliseconds)))
+
+(define (start-prime-test n start-time)
+  (if (prime? n)
+    (report-prime n (- (current-milliseconds) start-time))))
+
+(define (report-prime n elapsed-time)
+  (display n)
+  (display " *** ")
+  (display elapsed-time)
+  (newline))
+
+
+(define (search-for-primes min max)
+  (if (<= min max)
+      (cond ((odd? min) (timed-prime-test min)
+		        (search-for-primes (+ min 2) max))
+	    (else       (search-for-primes (+ min 1) max)))))
+
+
+; (search-for-primes 100000 100100): First 3 take 1 ms each
+; (search-for-primes 1000000 1000100): First 3 take 4, 3, 5 ms each
+; (search-for-primes 10000000 10000200): First 3 take 14, 12, 13 ms each
+
+; (sqrt 10) is 3.1622, so time increase is around sqrt(n), as expected.
+
+
+(define (next n)
+  (if (= n 2)
+      3
+      (+ n 2)))
+
+
+;(define (find-divisor n test-divisor)
+  ;(cond ((> (square test-divisor) n) n)
+	;((divides? test-divisor n) test-divisor)
+	;(else (find-divisor n (next test-divisor)))))
+
+; Now (search-for-primes 10000000 10000200): takes 7, 7, 10 ms each.  So approximately
+; half for the first two but a bit more for the third.  This will be quicker, if the
+; procedure has to search through lots of candidate divisors.  Each call to next is
+; more expensive that adding 1, which may account for the difference.
+
+; Increase the size of the integers (to reduce noisy measurements):
+; (search-for-primes 10000000000 10000000200)
+; This takes ~141ms for the +1 case and ~87ms for the next case: A speedup of around
+; 1.6.  This must be accounted for by the extra comparison.
+
+
+; 1.24
+
+(define (fermat-test n)
+  (define (try-it a)
+    (= (expmod a n n) a))
+  (try-it (+ 1 (pseudo-random-integer (- n 1)))))
+
+(define (expmod base exp m)
+  (cond ((= exp 0) 1)
+	((even? exp)
+	 (remainder (square (expmod base (/ exp 2) m))
+		    m))
+	(else
+	  (remainder (* (expmod base (- exp 1) m) base)
+		     m))))
+
+(define (fast-prime? n times)
+  (cond ((= times 0) #t)
+	((fermat-test n) (fast-prime? n (- times 1)))
+	(else #f)))
+
+(define (prime? n)
+  (fast-prime? n 10))
+
+; Now prime test as above takes 2-3 ms for the first three answers
+;  (search-for-primes (square 10000000000) (+ 500 (square 10000000000)))
+; This should be about double, given the the algorithm is logarithmic in time.
+; We get 6, 8, 8 ms (more like 3 times as long).  Maybe this is from the higher 
+; overhead of the random number generator?  Or more likely, because we are running
+; the Fermat test 10 times.
+
+; 1.25: The answer should be the same if we take remainders after exponentiating,
+; but exponentiating first keeps the numbers smaller, thus enabling testing of 
+; larger primes with less memory.
+
+; 1.26: Writing out the multiplication instead of using square means that the
+; argument is computed twice (once for each argument to * rather than once for 
+; square.  So we are no longer halving the number of steps needed.  Hence, this is
+; no longer a log n algorithm.
+
+; 1.27
+
+; Test if a^n is congruent to a mod n for all a<n
+
+(define (fermat n)
+  (define (fermat-rec a n)
+    (if (= a 1) 
+	#t
+	(and (= (expmod a n n) a)
+	     (fermat-rec (- a 1) n))))
+  (fermat-rec (- n 1) n))
+
+
diff --git a/1_25.sch b/1_25.sch
new file mode 100644
index 0000000..1cd1444
--- /dev/null
+++ b/1_25.sch
@@ -0,0 +1,14 @@
+(define (expmod base exp m)
+  (remainder (fast-expt base exp) m))
+
+(define (fast-expt b n)
+  (cond ((= n 0) 1)
+	((even? n) (square (fast-expt b (/ n 2))))
+	(else (* b (fast-expt b (- n 1))))))
+
+(define (even? n)
+  (= (remainder n 2) 0))
+
+; This appears to be equivalent to calculating the exponent modulo m with the
+; exception that for large numbers, fast-expt will get very large before 
+; calculating the modulus, so it is probably not suitable for a fast prime tester.
diff --git a/1_28.sch b/1_28.sch
new file mode 100644
index 0000000..dc4ee05
--- /dev/null
+++ b/1_28.sch
@@ -0,0 +1,44 @@
+(import chicken.random)  ; for pseudo-random-integer
+
+(define (square x) (* x x))
+
+(define (non-trivial-root? x n)
+  (and (not (= x (- n 1)))
+       (not (= x 1))
+       (= (remainder (square x) n) 1)))
+
+(define (expmod base exp m)
+  (cond ((= exp 0) 1)
+	((even? exp)
+	  (let ((test (expmod base (/ exp 2) m)))
+	    (if (non-trivial-root? test m)
+		0  ; Signal non-trivial root
+	        (remainder (square test)
+		           m))))
+	 (else
+	  (remainder (* (expmod base (- exp 1) m) base)
+		     m))))
+
+(define (miller-rabin-test n count)
+  (let* ((a (+ 1 (pseudo-random-integer (- n 1))))
+	 (result (expmod a (- n 1) n)))
+    ; (display a)
+    ; (newline)
+    (cond ((= count 0) 
+	      ; (display "Count 0")
+	      ; (newline)
+   	      #t)	; Exhausted our attempts.  Assumed prime
+	  ((= result 0) 
+	      ; (display "Non-trivial root")
+	      ; (newline)
+	      #f)	; Non-trivial root: not prime
+	  ((> result 1) 
+	      ; (display "MR test failed: ")
+	      ; (display result)
+	      ; (newline)
+	      #f)	; a^(n-1) not congruent 1 (n): not prime
+	  (else (miller-rabin-test n (- count 1))))))
+
+
+(define (prime? n)
+  (miller-rabin-test n 25))
diff --git a/1_29.sch b/1_29.sch
new file mode 100644
index 0000000..38a468a
--- /dev/null
+++ b/1_29.sch
@@ -0,0 +1,17 @@
+(define (cube x) (* x x x))
+
+(define (simpson-iter f a b n k)
+  (let* ((h (/ (- b a) n))
+	 (y (f (+ a (* k h))))
+	 (coeff 
+	  (cond ((= k 0) 1)
+		((= k n) 1)
+		((odd? k) 4)
+		((even? k) 2))))
+    (+ (* (/ h 3) (* coeff y))
+      (if (= k n)
+	0
+      	(simpson-iter f a b n (+ k 1))))))
+
+(define (simpson-int f a b n)
+  (simpson-iter f a b n 0))
diff --git a/1_30.sch b/1_30.sch
new file mode 100644
index 0000000..d5332bf
--- /dev/null
+++ b/1_30.sch
@@ -0,0 +1,13 @@
+(define (sum-iter term a next b)
+  (define (iter a result)
+    (if (> a b)
+	result
+	(iter (next a) (+ result (term a)))))
+  (iter a 0))
+
+(define (pi-sum a b)
+  (define (pi-term x)
+    (/ 1.0 (* x (+ x 2))))
+  (define (pi-next x)
+    (+ x 4))
+  (sum-iter pi-term a pi-next b))
diff --git a/1_31.sch b/1_31.sch
new file mode 100644
index 0000000..802262a
--- /dev/null
+++ b/1_31.sch
@@ -0,0 +1,33 @@
+(define (square x) (* x x))
+
+(define (product-rec term a next b)
+  (if (> a b)
+      1
+      (* (term a)
+	 (product term (next a) next b))))
+
+
+(define (product-iter term a next b)
+  (define (iter a result)
+    (if (> a b)
+	result
+	(iter (next a) (* result (term a)))))
+  (iter a 1))
+
+(define product product-iter)
+;(define product product-rec)
+
+(define (fact n)
+  (product
+    (lambda (x) x)
+    1
+    (lambda (x) (+ x 1))
+    n))
+
+; Approximation to pi/4
+(define (pi-prod n)
+  (product
+    (lambda (a) (/ (* a (+ a 2)) (square (+ a 1))))
+    2
+    (lambda (a) (+ a 2))
+    n))
diff --git a/1_32.sch b/1_32.sch
new file mode 100644
index 0000000..edd88b5
--- /dev/null
+++ b/1_32.sch
@@ -0,0 +1,41 @@
+(define (accumulate-rec combiner null-value term a next b)
+  (if (> a b)
+      null-value
+      (combiner (term a)
+		(accumulate combiner null-value term (next a) next b))))
+
+(define (accumulate-iter combiner null-value term a next b)
+  (define (iter a result)
+    (if (> a b)
+	result
+	(iter (next a) (combiner result (term a)))))
+  (iter a null-value))
+
+;(define accumulate accumulate-rec)
+(define accumulate accumulate-iter)
+
+(define (product term a next b)
+  (accumulate 
+    (lambda (x y) (* x y))
+    1
+    term
+    a
+    next
+    b))
+
+(define (sum term a next b)
+  (accumulate
+    (lambda (x y) (+ x y))
+    0
+    term
+    a
+    next
+    b))
+
+(define (fact n)
+  (product
+    (lambda (x) x)
+    1
+    (lambda (x) (+ x 1))
+    n))
+
diff --git a/1_33.sch b/1_33.sch
new file mode 100644
index 0000000..77c1304
--- /dev/null
+++ b/1_33.sch
@@ -0,0 +1,38 @@
+(load "1_28.sch")	; for prime?
+
+(define (filtered-accumulate combiner null-value pred term a next b)
+  (if (> a b)
+    null-value
+    (combiner
+      (if (pred a) (term a) null-value)
+      (filtered-accumulate combiner null-value pred term (next a) next b))))
+
+(define (square x) (* x x))
+
+(define (gcd a b)
+  (if (= b 0)
+    a
+    (gcd b (remainder a b))))
+
+(define (rel-prime a b)
+  (= (gcd a b) 1))
+
+(define (sum-squares-primes a b)
+  (filtered-accumulate
+    (lambda (x y) (+ x y))
+    0
+    prime?
+    square
+    a
+    (lambda (x) (+ 1 x))
+    b))
+
+(define (prod-rel-prime n)
+  (filtered-accumulate
+    (lambda (x y) (* x y))
+    1
+    (lambda (x) (rel-prime x n))
+    (lambda (x) x)
+    1
+    (lambda (x) (+ 1 x))
+    n))
diff --git a/1_35.sch b/1_35.sch
new file mode 100644
index 0000000..73cd34b
--- /dev/null
+++ b/1_35.sch
@@ -0,0 +1,15 @@
+(define tolerance 0.00001)
+
+(define (fixed-point f first-guess)
+  (define (close-enough? v1 v2)
+    (< (abs (- v1 v2)) tolerance))
+      (define (try guess)
+	(let ((next (f guess)))
+	  (if (close-enough? guess next)
+	    next
+	    (try next))))
+	  (try first-guess))
+
+(define (f x)
+  (+ 1 (/ 1 x)))
+
diff --git a/1_36.sch b/1_36.sch
new file mode 100644
index 0000000..78c54bb
--- /dev/null
+++ b/1_36.sch
@@ -0,0 +1,23 @@
+(define tolerance 0.00001)
+
+(define (fixed-point f first-guess)
+  (define (close-enough? v1 v2)
+    (< (abs (- v1 v2)) tolerance))
+  (define (try guess)
+    (let ((next (f guess)))
+      (display next)
+      (newline)
+      (if (close-enough? guess next)
+	next
+	(try next))))
+  (try first-guess))
+
+(define (f x)
+  (/ (log 1000) (log x)))
+
+(define (average x y)
+  (/ (+ x y) 2))
+
+; Using average damping, converges about 4 times quicker
+(define (f2 x)
+  (average x (f x)))
diff --git a/1_37.sch b/1_37.sch
new file mode 100644
index 0000000..fcea205
--- /dev/null
+++ b/1_37.sch
@@ -0,0 +1,21 @@
+(define (cont-frac-rec n d k)
+  (define (rec i)
+    (if (= i k)
+      (/ (n i) (d i))
+      (/ (n i) 
+	(+ (d i) (rec (+ i 1))))))
+  (rec 1))
+
+(define (cont-frac-iter n d k)
+  (define (iter i res)
+    (if (= i 0)
+      res
+      (iter (- i 1)
+	(/ (n i) (+ res (d i))))))
+  (iter k 0))
+
+
+(define phi (/ (+ 1 (sqrt 5)) 2))
+
+; 10 iterations needed to approximate phi using 
+; (lambda (i) 1.0) for n and d.
diff --git a/1_38.sch b/1_38.sch
new file mode 100644
index 0000000..f7769d5
--- /dev/null
+++ b/1_38.sch
@@ -0,0 +1,11 @@
+(load "1_37.sch")  ; for cont-frac-rec and cont-frac-iter
+
+(define (n i) 1.0)
+
+(define (d i)
+  (if (= (remainder i 3) 2) 
+      (* (+ (quotient i 3) 1) 2)
+      1))
+
+(define e
+  (+ 2 (cont-frac-rec n d 20)))
diff --git a/1_39.sch b/1_39.sch
new file mode 100644
index 0000000..8288456
--- /dev/null
+++ b/1_39.sch
@@ -0,0 +1,10 @@
+(load "1_37.sch") ; for cont-fram-rec
+
+(define (d i) (+ (* (- i 1) 2) 1))
+
+(define (tan-cf x)
+  (define (n i) 
+    (if (= i 1)
+      (* 1.0 x)
+      (* -1.0 (* x x))))
+  (cont-frac-rec n d 50))
diff --git a/1_40.sch b/1_40.sch
new file mode 100644
index 0000000..e69de29
diff --git a/1_8.sch b/1_8.sch
new file mode 100644
index 0000000..2bc131e
--- /dev/null
+++ b/1_8.sch
@@ -0,0 +1,16 @@
+(define (cbrt x)
+  (cbrt-iter 1.0 x))
+
+(define (cbrt-iter guess x)
+  (if (good-enough? guess x)
+      guess
+      (cbrt-iter (improve guess x) x)))
+
+(define (improve guess x)
+  (/
+   (+ (/ x (* guess guess)) (* 2 guess))
+   3))
+
+(define (good-enough? guess x)
+  (< (abs (- (* guess guess guess) x)) 0.001))
+
diff --git a/notes.txt b/notes.txt
new file mode 100644
index 0000000..1e73e86
--- /dev/null
+++ b/notes.txt
@@ -0,0 +1,86 @@
+1.9:
+
+(define (+ a b)
+ (if (= a 0) b (inc (+ (dec a) b))))
+
+(+ 4 5)
+(if (= 4 0) 4 (inc (+ (dec 4) 5)))
+(inc (+ 3 5))
+(inc (inc (+ 2 5)))
+(inc (inc (inc (+ 1 5))))
+(inc (inc (inc (inc (+ 0 5)))))
+(inc (inc (inc (inc 5))))
+(inc (inc (inc 6)))
+(inc (inc 7))
+(inc 8)
+9
+
+Recursive
+
+
+(define (+ a b)
+ (if (= a 0) b (+ (dec a) (inc b))))
+
+(+ 4 5)
+(+ 3 6)
+(+ 2 7)
+(+ 1 8)
+(+ 0 9)
+9
+
+Iterative
+
+
+1.10:
+
+(define (A x y)
+ (cond ((= y 0) 0)
+       ((= x 0) (+ 2 y))
+       ((= y 1) 2)
+       (else (A (- x 1)
+       	     	(A x (- y 1))))))
+
+(A 1 10)
+(A 0 (A 1 9))
+(A 0 (A 0 (A 1 8)))
+(A 0 (A 0 (A 0 (A 1 7))))
+(A 0 (A 0 (A 0 (A 0 (A 1 6)))))
+(A 0 (A 0 (A 0 (A 0 (A 0 (A 1 5))))))
+(A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 1 4)))))))
+(A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 1 3)))))))))
+(A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 1 2))))))))))
+(A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 1 1)))))))))))
+(A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 2))))))))))
+(A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 4))))))))))
+(A 0 (A 0 (A 0 (A 0 (A 0 (A 0 (A 0 6)))))))))
+(A 0 (A 0 (A 0 (A 0 (A 0 (A 0 8)))))))
+(A 0 (A 0 (A 0 (A 0 (A 0 10))))))
+(A 0 (A 0 (A 0 (A 0 12)))))
+(A 0 (A 0 (A 0 14))))
+(A 0 (A 0 16)))
+(A 0 18)
+22
+
+(A 2 4) = 16
+(A 3 3) = 16
+
+
+1.34
+
+(define (f g) (g 2))
+
+(f f) gives error Error: call of non-procedure: 2. This is because it is expecting the 
+second argument to be a procedure (implicitly from the fact that it applies it to 2).
+
+
+1.35
+
+Golden ratio: phi = (1 + sqrt(5))/2
+
+Let x' be a fixed point of x |-> 1 + 1/x.  Then
+
+x' = 1 + 1/x' = (x' + 1)/x'
+x'^2 = x' + 1
+x'^2 - x' - 1 = 0
+
+Solutions: (1 +- sqrt(1 + 4)) / 2.  So phi is one of the fixed points.