[ Diff(x[1], t) = ((-1)*k[13]*k[5]*x[1] + (-1)*k[13]*k[11]*k[10]*x[3]*x[1] + 1*k[13]*k[11]*x[2]*x[3]*x[1])/k[13], Diff(x[2], t) = ((-1)*k[13]*k[2]*x[2] + (-1)*k[13]*piecewise(t < 1, 1, piecewise(t > 15, 1, 21/5000))*k[8]*x[2]*x[3] + 1*k[13]*k[1])/k[13], Diff(x[3], t) = ((-1)*k[13]*k[3]*x[3] + 1*k[13]*piecewise(t < 1, 1, piecewise(t > 15, 1, 21/5000))*k[8]*x[2]*x[3] + (-1)*k[13]*k[9]*x[3]*x[4])/k[13], Diff(x[4], t) = ((-1)*k[13]*k[7]*x[4] + 1*k[13]*k[11]*k[10]*x[3]*x[1])/k[13] ]