[sage-devel] GiNaC as the symbolic manipulation engine in Sage

 
Using "sum"   as the key operation substituting for a loop of adding terms,
as well as the simplification of sums
is probably confusing.

probably we should advocate something like

for i:low thru hi add f(i)

rather than sum(f(i),i,low,hi).

Also note that there is a useful definition of sum(f(i),i,10,1),  and  it is
not sum(f(i),i,1,10);

It used to be, in macsyma...

SUMHACK default:FALSE

If set to TRUE, then SUM(F(I),I,3,1); will yield -F(2), by the
identity SUM(F(I),I,A,B) = - SUM(F(I),I,B+1,A-1) when A>B.




> -----Original Message-----
> From: maxima-bounces at math.utexas.edu 
> [mailto:maxima-bounces at math.utexas.edu] On Behalf Of Andrej Vodopivec
> Sent: Friday, August 15, 2008 7:48 AM
> To: Barton Willis
> Cc: maxima at math.utexas.edu; Robert Dodier
> Subject: Re: [Maxima] [sage-devel] GiNaC as the symbolic 
> manipulation engine in Sage
> 
> 2008/8/12 Barton Willis <willisb at unk.edu>:
> > -----maxima-bounces at math.utexas.edu wrote: -----
> >
> >>In the case of the summation problem mentioned in this thread,
> >>the central problem is that Maxima's simplification algorithm
> >>for the result (something of the form (x + y)^2 + (x + y)^4 + ...)
> >>seems to run in n^2 time where n is the number of terms.
> >
> > Simplus doesn't take advantage of sorting (sorting places common
> > terms next to each other); so
> >
> > (%i2) tree_reduce("+", makelist((x+y)^(2*i),i,1,10000))$
> > Evaluation took 0.4000 seconds (0.4000 elapsed)
> >
> > Here's a simple alternative simplus that makes xreduce faster
> > than tree_reduce for this sum: (simplus-prosaic is not in Maxima)
> >
> > (%i3) load("/mysimp/simplus-prosaic.o")$
> >
> > (%i4) xreduce("+", makelist((x+y)^(2*i),i,1,10000))$
> > Evaluation took 0.3700 seconds (0.3700 elapsed)
> >
> > Back to a fresh maxima:
> >
> > (%i3) xreduce("+", makelist((x+y)^(2*i),i,1,10000))$
> > Evaluation took 31.5000 seconds (31.5000 elapsed)
> 
> Maybe the sum algorithm (dosum in asum.lisp) should be improved.
> Something like my_sum should not be to hard to implement:
> 
> (%i1) my_sum(expr, i, lo, hi) :=
>     if hi=lo then subst(i=lo, expr)
>     else if hi-lo=1 then subst(i=lo, expr) + subst(i=hi, expr)
>     else block(
>         [lo1 : floor((hi+lo)/2)],
>         my_sum(expr, i, lo, lo1) + my_sum(expr, i, lo1+1, hi)
>     )$
> (%i2) showtime:all$
> Evaluation took 0.0000 seconds (0.0000 elapsed)
> 
> (%i3) tree_reduce("+", makelist((x+y)^(2*i), i, 1, 10000))$
> Evaluation took 0.5510 seconds (0.5560 elapsed)
> 
> (%i4) my_sum((x+y)^(2*i), i, 0, 10000)$
> Evaluation took 0.9770 seconds (0.9820 elapsed)
> 
> (%i5) compile(my_sum)$
> (%i6) my_sum((x+y)^(2*i), i, 0, 10000)$
> Evaluation took 0.3410 seconds (0.3490 elapsed)
> 
> (%i7) sum((x+y)^(2*i), i, 0, 10000)$
> Evaluation took 57.7640 seconds (57.9480 elapsed)
> 
> (%i9) %o6-%o7;
> Evaluation took 118.2390 seconds (118.9120 elapsed)
> (%o9) 0
> 
> Of course %o9 is bad too.
> 
> -- 
> Andrej
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