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Find largest left truncatable prime in a given base: Difference between revisions
Find largest left truncatable prime in a given base (view source)
Revision as of 14:56, 29 April 2013
, 11 years agoAdded Racket version
(Added Racket version) |
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</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}}==
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