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Laplace transform of Piecewise doesn't work even though SymPy can compute the integral #10354

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asmeurer opened this Issue Jan 4, 2016 · 6 comments

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asmeurer commented Jan 4, 2016

Reported on Gitter:

In [27]: u2=Piecewise((1.,t<1),(0.5,t>=1))

In [28]: print(laplace_transform(u2, t, s))
LaplaceTransform(Piecewise((1.00000000000000, t < 1), (0.500000000000000, t >= 1)), t, s)

In [29]: laplace_transform(u2, t, s).rewrite(Integral).doit()
Out[29]:
           -s
1.0   0.5⋅ℯ
─── - ───────
 s       s

@asmeurer asmeurer added the integrals label Jan 4, 2016

@ronanpaixao

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ronanpaixao commented Jan 5, 2016

Got a different, still unevaluated result:

In[34]: u2=Piecewise((1.,t<1),(0.5,t>=1))

In[35]: print(laplace_transform(u2, t, s))
LaplaceTransform(Piecewise((1.00000000000000, t < 1), (0.500000000000000, t >= 1)), t, s)

In[36]: laplace_transform(u2, t, s).rewrite(Integral).doit()
Out[36]: Integral(Piecewise((1.0*exp(-s*t), t < 1), (0.5*exp(-s*t), t >= 1)), (t, 0, oo))

In[37]: sympy.__version__
Out[37]: '0.7.6.1'
@asmeurer

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asmeurer commented Jan 5, 2016

I ran mine on master so that's probably why.

@nishnik

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nishnik commented Mar 9, 2016

@asmeurer You got the right results. And it is not coming the same when I tried to replicate it:

In [3]: u2=Piecewise((1.,t<1),(0.5,t>=1))

In [4]: laplace_transform(u2, t, s).rewrite(Integral).doit()
Out[4]: Integral(Piecewise((1.0*exp(-s*t), t < 1), (0.5*exp(-s*t), t >= 1)), (t, 0, oo))

In [5]: sympy.__version__
Out[5]: '1.0.1.dev'

What needs to be done here ?

@asmeurer

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asmeurer commented Mar 10, 2016

It isn't working for me anymore either. Something recent must have broken it.

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asmeurer commented Mar 10, 2016

I didn't try super hard, but I can't find a commit where it works. Not sure what happened. Is the answer I "got" above correct?

@nishnik

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nishnik commented Mar 10, 2016

Yeah! That's correct.
Then, let me try git bisect again and see if I can find which commit broke it.

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