Thanks Elijah that works really well. I’m looking to extend this to primes in other bases and try to construct and discover cases where both left and right truncation takes place simultaneously.
> On 19 Nov 2022, at 22:04, Elijah Stone <elro...@elronnd.net> wrote: > > x: ". y where y is simply a string of digits will interpret those digits as > a (fixed-width) integer and _then_ convert the latter to extended-precision. > You could append an 'x', or perhaps consider the following definition: > > truncs=. [:~. [:10&#.\. 10&#.^:_1 NB.equivalent to ltrunc^:a: > ,.truncs 357686312646216567629137x > 357686312646216567629137 > 57686312646216567629137 > 7686312646216567629137 > 686312646216567629137 > 86312646216567629137 > 6312646216567629137 > 312646216567629137 > 12646216567629137 > 2646216567629137 > 646216567629137 > 46216567629137 > 6216567629137 > 216567629137 > 16567629137 > 6567629137 > 567629137 > 67629137 > 7629137 > 629137 > 29137 > 9137 > 137 > 37 > 7 > >> On Sat, 19 Nov 2022, Richard Donovan wrote: >> >> Hi >> >> I am doing experiments with large primes, in particular looking at primes >> that remain primes when n digits are truncated from the left. For example >> 6391373 391373 91373 1373 373 73 3 remains prime at each >> step. >> >> The largest of these in base 10 is 357686312646216567629137. >> >> I wrote the following code in preparation for further investigation but I >> find that the 24 digit number above is not handled as I wish it to be, as >> you will see below. >> >> What have I missed? >> >> Thanks >> >> >> >> >> digits >> "."0@": >> >> ltrunc >> 3 : 0"0 0 0 >> n=: ": 0, }. digits y >> x: ". n-. ' ' >> ) >> >> >> >> NB. Works fine with 7 digit number... >> ltrunc^:a: 6391373 >> 6391373 391373 91373 1373 373 73 3 0 >> >> >> NB. But I can’t get it working for a 24 digit number! >> ltrunc 357686312646216567629137 >> 0 0 5 7 6 8 6 0 2 3 >> ltrunc 357686312646216567629137x >> 57686312646216568012800 >> ltrunc x:357686312646216567629137x >> 57686312646216568012800 >> >> Is there a way around the limit? >> >> ---------------------------------------------------------------------- >> For information about J forums see http://www.jsoftware.com/forums.htm > ---------------------------------------------------------------------- > For information about J forums see http://www.jsoftware.com/forums.htm ---------------------------------------------------------------------- For information about J forums see http://www.jsoftware.com/forums.htm
