Wronskian

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Let $\mathbb{T}$ be a time scale. Consider the dynamic equation $$y^{\Delta \Delta} + p(t)y^{\Delta} + q(t) y = f(t),$$ where $p,q,$ and $f$ are rd-Continuous. For two $\Delta$-differentiable functions $y_1$ and $y_2$ define the Wronskian $W=W(y_1,y_2)$ by $$W(t) = \mathrm{det} \left[ \begin{array}{ll} y_1(t) & y_2(t) \\ y_1^{\Delta}(t) & y^{\Delta}_2(t) \end{array} \right].$$