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wrong answer from solve?

asked 2016-01-22 10:03:04 -0600

beeson gravatar image
var('t')
test3 = -4*cos(t)^4 + 4*cos(t)^2 - 1
sage: solve(test3==0,t)
[]

When actually the equation is equivalent to (cos(t))^2 = 1/2 and it does have solutions.

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answered 2016-01-22 12:36:37 -0600

kcrisman gravatar image
sage: solve(test3==0,t,to_poly_solve=True)
[t == 1/4*pi + 1/2*pi*z35]

You do have to read the documentation of solve? to see this option. It's not default because sometimes it will not return exact solutions.

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So then, if solve returns [], that does not mean "there are no solutions". It means, "I didn't find any solutions". Similarly, if it does return a list of solutions, I guess that means "Here are the solutions I found", rather than, "Here are all the solutions." If this is the case, the documentation should say so up front. It says, "if there are no solutions it returns an empty list" but says nothing about the converse.

beeson gravatar imagebeeson ( 2016-01-22 13:54:33 -0600 )edit

I think it does say something along these lines when it talks about to_poly_solve, but we would welcome specific suggestions for how to improve the documentation, for sure!

kcrisman gravatar imagekcrisman ( 2016-01-26 11:21:28 -0600 )edit

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Asked: 2016-01-22 10:03:04 -0600

Seen: 79 times

Last updated: Jan 22 '16