desolve fails when assumptions required

[sagetrac-descopau]

I want to solve f''/f=k with k in R

sage:  x=var('x')
sage: f=function('f',x)
sage: k=var('k')
sage: assume(k>0)
sage:  desolve(diff(f(x),x,2)/f(x)==k,[f,x]) 

and sage keeps answering :

TypeError                                 Traceback (most recent call last)

/home/moi/<ipython console> in <module>()

/home/moi/sage-4.4.1-linux-32bit-ubuntu_9.10-i686-Linux/local/lib/python2.6/site-packages/sage/calculus/desolvers.pyc in desolve(de, dvar, ics, ivar, show_method, contrib_ode)
    338     # we produce string like this
    339     # ode2('diff(y,x,2)+2*'diff(y,x,1)+y-cos(x),y(x),x)
--> 340     soln = maxima(cmd)
    341 
    342     if str(soln).strip() == 'false':

/home/moi/sage-4.4.1-linux-32bit-ubuntu_9.10-i686-Linux/local/lib/python2.6/site-packages/sage/interfaces/expect.pyc in __call__(self, x, name)
   1030             
   1031         if isinstance(x, basestring):
-> 1032             return cls(self, x, name=name)
   1033         try:
   1034             return self._coerce_from_special_method(x)

/home/moi/sage-4.4.1-linux-32bit-ubuntu_9.10-i686-Linux/local/lib/python2.6/site-packages/sage/interfaces/expect.pyc in __init__(self, parent, value, is_name, name)
   1449             except (TypeError, KeyboardInterrupt, RuntimeError, ValueError), x:
   1450                 self._session_number = -1
-> 1451                 raise TypeError, x
   1452         self._session_number = parent._session_number
   1453 

TypeError: Computation failed since Maxima requested additional constraints (try the command 'assume(k>0)' before integral or limit evaluation, for example):
Is  k  positive, negative, or zero?

so I tried :

sage:  desolve(diff(f(x),x,2)/f(x)==k^2+1,[f,x])

and I got :

k1*e^(I*sqrt(-k^2 - 1)*x) + k2*e^(-I*sqrt(-k^2 - 1)*x)

!!

CC: @robert-marik @kcrisman

Component: symbolics

Keywords: desolve

Author: Flavia Stan

Reviewer: Karl-Dieter Crisman

Merged: sage-4.7.alpha4

Issue created by migration from https://trac.sagemath.org/ticket/8931

[sagetrac-monkey]

comment:1

I have the same problem solving a different equation:

var('L1 L2 C1 C2 L3 C3 t')
forget()
assume(L2 > 0)
assume(L3 > 0)
assume(C2 > 0)
assume(C3 > 0)
j = function('j',t)
eqn = diff(v_i,t) - L2*diff(j,t,2) - j/C2 == L3*diff(j,t,2) + j/C3
desolve(eqn,[j,t],[0,1])

returns

Traceback (click to the left of this block for traceback)
...
Is  C2*C3*(C3+C2)*(L3+L2)  positive, negative, or zero?

The only quick workaround I have found is to specify the values of the constants, but I really need them to be symbolic. The best I can do is use primes for values of constants and after several such sets you can get a picture of how the constants are related to the numbers in the solution terms, but it's an ass-ache.

I think the best solution would be to have an optional argument to desolve which answers the question posed if there are not sufficient assumptions/conditions to already determine the correct answer.

Really appreciate the great work so far guys! :D

[robert-marik]

comment:2

with #9835

marik@um-bc107:/opt/sage$ ./sage
----------------------------------------------------------------------
| Sage Version 4.5.3, Release Date: 2010-09-04                       |
| Type notebook() for the GUI, and license() for information.        |
----------------------------------------------------------------------
sage: x=var('x'); f=function('f',x); k=var('k'); assume(k>0)
sage: desolve(diff(f,x,2)/f==k,f,ivar=x)
k1*e^(sqrt(k)*x) + k2*e^(-sqrt(k)*x)

[sagetrac-fstan]

Attachment: trac_8931_doctest.patch.gz

[sagetrac-fstan]

comment:3

The patch adds a doc test for this issue.

[kcrisman]

Reviewer: Karl-Dieter Crisman

[kcrisman]

Changed author from G.Connan to Flavia Stan

[kcrisman]

comment:5

Applies cleanly to 4.7.alpha1, passes tests, clearly fixes the issue, looks good in the documentation. Positive review.

Not sure what to do on the author; fstan is the person I have put, as the person who posted the patch, but before it had G. Cannon. Unless fstan comments on this, let's leave her as the author.

[jdemeyer]

Merged: sage-4.7.alpha4