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2014-10-24 17:16:29 +0100 asked a question 2nd-order ODE: Maxima wants sign of 1 constant before finishing

I'm trying to solve a 2nd-order nonlinear ODE using desolve(). Maxima gets partway through, but then asks for the sign of one of the integration constants. How can I tell it to assume something that doesn't exist before I call desolve()?

t,P,x0=var('t,P,x0')
x=function('x',t)
de=(diff(x,t,2)*x^2+P==0)
forget()
assume(P>0,x0>0)
desolve(de,x,ics=[0,x0,0],ivar=t)

gives the error:

TypeError: ECL says: Maxima asks: Is  %k1  positive or negative?