hi could you please tell me what's wrong with this code?

I get this error " Warning: Explicit solution could not be found.

>> syms x(t) y(t)

z=dsolve(diff(y)==x-x.^3,diff(x)==y)

[Alberto]

It happends the same with a more simplest version:

syms x(t)

z=dsolve(diff(diff(x))==x.^3 )

[Walter Roberson]

That's a messy pair. The solution appears to be

y(t) = diff(x(t),t)
x(t) = C2 * sqrt(2) * sqrt(1/(C2^2+1)) * JacobiSN(((1/2*i) * sqrt(2) * t + C1) * sqrt(2) * sqrt(1/(C2^2+1)), C2)

where C1 and C2 are constants of proportionality