It seems that the answer about the sign of la is kept somewhere,
while it shouldn't be kept. This information can be kicked out by
additional efforts:
Maxima 5.9.1 http://maxima.sourceforge.net
Using Lisp Kyoto Common Lisp GCL 2.6.5 (aka GCL)
Distributed under the GNU Public License. See the file COPYING.
Dedicated to the memory of William Schelter.
This is a development version of Maxima. The function bug_report()
provides bug reporting information.
(%i1) integrate(EXP(-la*t)*la,t,0,INF);
Is la positive, negative, or zero?
n;
Integral is divergent
-- an error. Quitting. To debug this try DEBUGMODE(TRUE);
(%i2) integrate(EXP(-la*t)*la,t,0,INF);
Integral is divergent
-- an error. Quitting. To debug this try DEBUGMODE(TRUE);
HOW DOES IT KNOW ?
(%i3) is(la<0);
MACSYMA was unable to evaluate the predicate:
la < 0
-- an error. Quitting. To debug this try DEBUGMODE(TRUE);
(%i4) integrate(EXP(-la*t)*la,t,0,INF);
Integral is divergent
-- an error. Quitting. To debug this try DEBUGMODE(TRUE);
HOW DOES IT KNOW ?
(%i5) is(la>0);
MACSYMA was unable to evaluate the predicate:
la > 0
-- an error. Quitting. To debug this try DEBUGMODE(TRUE);
(%i6) integrate(EXP(-la*t)*la,t,0,INF);
Integral is divergent
-- an error. Quitting. To debug this try DEBUGMODE(TRUE);
HOW DOES IT KNOW ?
(%i7) is(la=0);
(%o7) FALSE
NOW THE INFO IS KICKED OUT
(%i8) integrate(EXP(-la*t)*la,t,0,INF);
Is la positive, negative, or zero?
p;
(%o8) 1