How to specify proprties of a constant

[maarte...@knmi.nl]

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

[Aaron Meurer]

Use 

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

Aaron Meurer 

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[maarte...@knmi.nl]

On Thursday, September 22, 2016 at 6:24:39 PM UTC+2, Aaron Meurer wrote:

Use 

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

Thank you.