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diff --git a/net/ricketyspace/sicp/one/twentyseven.scm b/net/ricketyspace/sicp/one/twentyseven.scm
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+;;;; Under Creative Commons Attribution-ShareAlike 4.0
+;;;; International. See
+;;;; <https://creativecommons.org/licenses/by-sa/4.0/>.
+
+(define-module (net ricketyspace sicp one twentyseven)
+  #:use-module (srfi srfi-1)
+  #:export (carmichael-numers-fool-fermat-test?))
+
+(define carmichael-numbers '(561 1105 1729 2465 2821 6601))
+
+(define (square x) (* x x))
+
+(define (expmod base exp m)
+  (cond ((= exp 0) 1)
+        ((even? exp)
+         (remainder (square (expmod base (/ exp 2) m))
+                    m))
+        (else
+         (remainder (* base (expmod base (- exp 1) m))
+                    m))))
+
+(define (fermat-test n)
+  (define (try-it a)
+    (= (expmod a n n) a))
+  (try-it (+ 1 (random (- n 1)))))
+
+(define (fast-prime? n times)
+  (cond ((= times 0) #t)
+        ((fermat-test n) (fast-prime? n (- times 1)))
+        (else #f)))
+
+(define (carmichael-numers-fool-fermat-test?)
+  "Returns #t if all `carmichael-numbers` pass (fool) the fermat test."
+  (let* ((fooled-test? (lambda (n) (fast-prime? n 100000)))
+        (fooled? (lambda (result) (eq? result #t)))
+        (results (map fooled-test? carmichael-numbers)))
+    (every fooled? results)))