[Maxima] behaviour of ev()
fateman at cs.berkeley.edu
Wed Apr 29 17:44:43 CDT 2009
Here's a start.
Never use ev.
Use subst instead.
Only if you cannot use subst for what you want to do, should you
consider using ev.
The reason is: ev is elaborate, and does many things that are not
You probably mean to say ev(s,r=a), since you probably don't want to
set the value of r permanently.
but even so, using subst is cleaner.
Al Berich wrote:
> I do not have much experience with maxima and still find plenty of
> things, which I do not understand:
> When including the function in line %i39 in an ev() and defining r:a
> thus log(r/a)=log(a/a)=0, apparently also 4*c4*r is set zero...
> A work around (storing %i39 in s) and evaluating ev(s,r:a) yields the
> expected result (%o42).
> I am confused...
> (%i37) kill(r,w,a,c1,c2,c3,c4);
> (%o37) done
> (%i38) w: c1+c2*r^2+c3*log(r/a)+c4*r^2*log(r/a)+w1;
> p r 2 r r 2
> (%o38) ---- + c4 r log(-) + c3 log(-) + c2 r + c1
> 64 K a a
> (%i39) s: ratexpand(diff(diff(w,r)*r,r));
> p r r
> (%o39) ---- + 4 c4 r log(-) + 4 c4 r + 4 c2 r
> 4 K a
> (%i40) ev(ratexpand(diff(diff(w,r)*r,r)),r:a);
> a p
> (%o40) ---- + 4 a c2
> 4 K
> (%i42) ev(s,r:a);
> a p
> (%o42) ---- + 4 a c4 + 4 a c2
> 4 K
> thank you in advance for help
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