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We are concerned with the problem of differentiability of the derivatives of order $m+1$ of solutions to the “nonlinear basic systems” of the type $${(-1)}^m\sum _{\left|\alpha \right|=m}{D}^{\alpha }{A}^{\alpha }\left(D^{\left(m\right)}u\right)+\frac{\partial u}{\partial t}=0\text{in}Q.$$ We are able to show that $$D^{\alpha }u\in L^2(-a,0,H^{\vartheta }(B\left(\sigma \right),{ℝ}^N)),|\alpha |=m+1,$$ for $\vartheta \in (0,1/2)$ and this result suggests that more regularity is not expectable.