http://www.perlmonks.org?node_id=1067582

slinky773 has asked for the wisdom of the Perl Monks concerning the following question:

On the TI-Nspire calculator, every division answer given, unless irrational, is given in exact form, either as an integer or as a fraction. I can do this in Perl with the fraction module. Every square root answer given is also given in exact form, either as an integer or as a radical. I do not know how to do this in perl. How do you convert a number to a radical?

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Re: Converting a Number into a Radical
by syphilis (Archbishop) on Dec 18, 2013 at 05:50 UTC
    I gather we're concerned only with square roots of integers and that, for example, if the answer is sqrt(18), you want to see it presented as 3 x sqrt(2) instead of simply sqrt(18).
    To achieve this you'll first want to factorise the integer - you could use something like Math::Factor::XS or Math::Prime::Util.

    To stick with the above example, 18 factorises to the primes 2,3,3 - multiply those primes together and you end up with 18.
    Notice that the "3" occurs twice - hence you can write the solution as 3 * sqrt(2).
    For sqrt(1050) - the prime factors are 2,3,5,5,7, with the "5" occurring twice, and the solution is therefore: 
    5 * sqrt(2 * 3 * 7) = 5 * sqrt(42)

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    That's essentially how it's done - I don't know if there's a module that does it all for you, or whether you'll have to program a good portion of it yourself. (Perhaps Math::NumSeq might also have something to offer.)

    Cheers,
    Rob
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      Thanks, Rob. That actually really helps.
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      I could not resist to write it out: 

      use strict;
use warnings;
use Math::Factor::XS 'prime_factors';

my $n = shift;
( $n and $n > 0 and $n == int $n ) or die "Need positive integer as in
+put!\n";

my %p;
$p{$_}++ for prime_factors( $n );

my $radical = 1;
$radical *= $_ for grep { $p{$_}%2 } keys %p;
print "sqrt( $n ) = ", sqrt( $n/$radical ), " * sqrt( $radical )\n";

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      What I was missing is a product function, like sum from List::Util. There is one in the replacement List::Util that comes with Scalar::Util but that means overwriting the standard module which makes me feel uncomfortable.

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      Awesome, syphilis - I wish it were possible to vote it up by more than than one.

      A user level that continues to overstate my experience :-))
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Re: Converting a Number into a Radical
by Anonymous Monk on Dec 18, 2013 at 04:49 UTC

    I don't know of a Perl solution, but SymPy (see www.sympy.org) can do this kind of thing.

    >>> sqrt(49)
7
>>> sqrt(27)
3*sqrt(3)
>>> simplify(sqrt(4+sqrt(448)))
2*sqrt(1 + 2*sqrt(7))

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