id,summary,reporter,owner,description,type,status,priority,milestone,component,resolution,keywords,cc,merged,author,reviewer,upstream,work_issues,branch,commit,dependencies,stopgaps
12809,Solve does not give consistent results when a dummy variable is involved,afleckenstein,burcin,"When working on another ticket (#11201) that involves documentation for the solve function, doing doctests proved to be frustrating. I added to the examples section of the docstring, and then multiple tests failed. I did some manual testing in sage and noticed a problem:
{{{
sage: var('x y')
(x, y)
sage: solve(cos(x)*sin(y)==1/2, x+y==0,x,y)
[[x == 1/4*pi + pi*z6, y == -1/4*pi - pi*z6]]
}}}
This is a perfectly valid answer, except that when I tried to solve the same equations again, the answer is different:
{{{
sage: solve(cos(x)*sin(y)==1/2, x+y==0,x,y)
[[x == 1/4*pi + pi*z14, y == -1/4*pi - pi*z14]]
}}}
This is still mathematically correct.
If I solve them many more times, z's coefficient keeps going up by 8:
{{{
sage: solve(cos(x)*sin(y)==1/2, x+y==0,x,y)
[[x == 1/4*pi + pi*z22, y == -1/4*pi - pi*z22]]
sage: solve([cos(x)*sin(x) == 1/2, x+y == 0],x,y)
[[x == 1/4*pi + pi*z30, y == -1/4*pi - pi*z30]]
sage: solve([cos(x)*sin(x) == 1/2, x+y == 0],x,y)
[[x == 1/4*pi + pi*z38, y == -1/4*pi - pi*z38]]
sage: solve([cos(x)*sin(x) == 1/2, x+y == 0],x,y)
[[x == 1/4*pi + pi*z46, y == -1/4*pi - pi*z46]]
sage: solve([cos(x)*sin(x) == 1/2, x+y == 0],x,y)
[[x == 1/4*pi + pi*z54, y == -1/4*pi - pi*z54]]
sage: solve([cos(x)*sin(x) == 1/2, x+y == 0],x,y)
[[x == 1/4*pi + pi*z62, y == -1/4*pi - pi*z62]]
sage: solve([cos(x)*sin(x) == 1/2, x+y == 0],x,y)
[[x == 1/4*pi + pi*z70, y == -1/4*pi - pi*z70]]
sage: solve([cos(x)*sin(x) == 1/2, x+y == 0],x,y)
[[x == 1/4*pi + pi*z78, y == -1/4*pi - pi*z78]]
sage: solve([cos(x)*sin(x) == 1/2, x+y == 0],x,y)
[[x == 1/4*pi + pi*z86, y == -1/4*pi - pi*z86]]
sage: solve([cos(x)*sin(x) == 1/2, x+y == 0],x,y)
[[x == 1/4*pi + pi*z94, y == -1/4*pi - pi*z94]]
}}}
All still mathematically correct.
This is a problem when you are trying to add examples to the documentation for solve and many doctests fail because the coefficients for the dummy variable are shifted up by some integer amount. I have included the file expressions_pyx_doctests.txt to show more of what I am talking about. To cause this, I added two examples, and the examples that I added were not complained about in the doctest. Also, z's coefficient went up by 6 this time, different than 8 in the first example.
",defect,new,minor,sage-6.4,symbolics,,solve,kcrisman mjo eviatarbach,,,,N/A,,,,,