Use

m, k = symbols("m, kappa", positive=True)

Aaron Meurer

On Thursday, September 22, 2016, <maarten.sn...@knmi.nl> wrote:

> Hi,
>
> I'm creating an example for using SymPy, using the good old harmonic
> oscillator as an example:
>
> import sympy
> s = sympy.Function("s")
> t, zeta, m, k = sympy.symbols("t, zeta, m, kappa")
> expr = sympy.Eq(m*s(t).diff(t, t), -k*s(t))
> sympy.dsolve(expr, s(t), ics={s(t).diff(t,1).subs(t,0):0})
>
>
> The latex form of the output is:
>
> s{\left (t \right )} =
>   C_{1} e^{- t \sqrt{- \frac{\kappa}{m}}} +
>   C_{2} e^{t \sqrt{- \frac{\kappa}{m}}}
>
>
> This gives a nice and generic result, but a bit too generic to my taste.
> For instance the constants m and k are both positive, and with that
> knowledge it is easy to express teh result using sine and cosine functions
> (and with the constraint: only a cosine).
>
> How do I tell SymPy that m and k are > 0?
>
> Best,
>
> Maarten
>
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