Find largest left truncatable prime in a given base: Difference between revisions

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@@ Line 629: / Line 629: @@
 </pre>
 That is going to be big!
+=={{header|Racket}}==
+<lang racket>
+#lang racket
+(require math/number-theory)
+(define (prepend-digit b d i n)
+  (+ (* d (expt b i)) n))
+(define (extend b i ts)
+  (define ts*
+    (for/list ([t (in-set ts)])
+           (for/set ([d (in-range 1 b)]
+                     #:when (prime? (prepend-digit b d i t)))
+                    (prepend-digit b d i t))))
+  (apply set-union ts*))
+(define (truncables b n)
+  ; return set of truncables of length n in base b
+  (if (= n 1)
+      (for/set ([d (in-range 1 b)] #:when (prime? d)) d)
+      (extend b (- n 1) (truncables b (- n 1)))))
+(define (largest b)
+  (let loop ([ts (truncables b 1)]
+             [n 1])
+    (define ts* (extend b n ts))
+    (if (set-empty? ts*)
+        (apply max (set->list ts))
+        (loop ts* (+ n 1)))))
+(for/list ([b (in-range 3 18)])
+  (define l (largest b))
+  ; (displayln (list b l))
+  (list b l))
+; Output:
+'((3 23)
+  (4 4091)
+  (5 7817)
+  (6 4836525320399)
+  (7 817337)
+  (8 14005650767869)
+  (9 1676456897)
+  (10 357686312646216567629137)
+  (11 2276005673)
+  (12 13092430647736190817303130065827539)
+  (13 812751503)
+  (14 615419590422100474355767356763)
+  (15 34068645705927662447286191)
+  (16 1088303707153521644968345559987)
+  (17 13563641583101))
+</lang>
 =={{header|Tcl}}==