Let G be an abelian group, let M2(C)\documentclass[12pt]{minimal}
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\begin{document}$$M_{2}(\mathbb {C})$$\end{document} be the algebra of complex 2×2\documentclass[12pt]{minimal}
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\begin{document}$$2\times 2$$\end{document} matrices, and let φ:G→G\documentclass[12pt]{minimal}
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\begin{document}$$\varphi :G\rightarrow G$$\end{document} be an endomorphism that need not be involutive. We determine the solutions Φ:G→M2(C)\documentclass[12pt]{minimal}
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\begin{document}$$\Phi :G\rightarrow M_{2}(\mathbb {C})$$\end{document} of the matrix functional equation Φ(x+y)+Φ(x+φ(y))2=Φ(x)Φ(y),x,y∈G.\documentclass[12pt]{minimal}
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\begin{document}$$\begin{aligned} \frac{\Phi (x+y)+\Phi (x+\varphi (y))}{2}=\Phi (x)\Phi (y),\quad x,y\in G. \end{aligned}$$\end{document}This enables us to characterize the solutions g:G→C2\documentclass[12pt]{minimal}
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\begin{document}$$g:G\rightarrow \mathbb {C}^{2}$$\end{document} and Φ:G→M2(C)\documentclass[12pt]{minimal}
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\begin{document}$$\Phi :G\rightarrow M_{2}(\mathbb {C})$$\end{document} of the following functional equation g(x+y)+g(x+φ(y))=2Φ(y)g(x),x,y∈G,\documentclass[12pt]{minimal}
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\begin{document}$$\begin{aligned} g(x+y)+g(x+\varphi (y))=2\Phi (y)g(x),\quad x,y\in G, \end{aligned}$$\end{document}under the invariant condition Φ∘φ=Φ\documentclass[12pt]{minimal}
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\begin{document}$$\Phi \circ \varphi =\Phi $$\end{document}.
