based on Raul's rosettacode link, but building an expaning list of left
truncable primes, such that further search is possible on future "iterations"
selPrime=: #~ 1&p:
ltrunc =: (] , ((1+i.9) (10 #. ,)"0 1 (10&#.inv))"0(,@:)(selPrime@:) each@:{:)
# &> sofar =: ltrunc^:6 < 3 7x
2 11 39 99 192 326 429
# &> sofar =: ltrunc^:6 sofar
2 11 39 99 192 326 429 521 545 517 448 354 276
the number of truncable primes decreases substantially as the digits increase.
(last answer is for 1 to 13 digits, with single digit primes wrongly listed (4
is right number of single digit primes)
the full list eventually goes to 0
# &> sofar =: ltrunc^:1 sofar (NB. iterations skipped)
2 11 39 99 192 326 429 521 545 517 448 354 276 212 117 72 42 24 13 6 5 4 3 1 0
which I guess we knew from example number
> _2 { sofar
357686312646216567629137
On Saturday, November 19, 2022 at 07:04:25 p.m. EST, Raul Miller
<[email protected]> wrote:
On Sat, Nov 19, 2022 at 6:47 PM 'Skip Cave' via Programming
<[email protected]> wrote:
> Now what is the J verb that will find an n-digit integer that is still
> prime when each of the digits are removed?
I'd probably go with https://rosettacode.org/wiki/Truncatable_primes#J for that.
Thanks,
--
Raul
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