The theory of gauge fields in Theoretical Physics poses several mathematical problems of interest in Differential Geometry and in Field Theory. Below, we tackle one of these problems: the existence of a finite system of generators of gauge-invariant Lagrangians and how to compute them. More precisely, if p:C→M\documentclass[12pt]{minimal}
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\begin{document}$$p:C\rightarrow M$$\end{document} is the bundle of connections on a principal G-bundle π:P→M\documentclass[12pt]{minimal}
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\begin{document}$$\pi :P\rightarrow M$$\end{document}, and G is a semisimple connected Lie group, then a finite number L1,⋯,LN′\documentclass[12pt]{minimal}
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\begin{document}$$L_1,\dotsc ,L_{N^\prime }$$\end{document} of gauge-invariant Lagrangians defined on J1C\documentclass[12pt]{minimal}
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\begin{document}$$J^1C$$\end{document} is proved to exist such that for any other gauge-invariant Lagrangian L∈C∞(J1C)\documentclass[12pt]{minimal}
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\begin{document}$$L\in C^\infty (J^1C)$$\end{document}, there exists a function F∈C∞(RN′)\documentclass[12pt]{minimal}
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\begin{document}$$F\in C^\infty ({\mathbb {R}}^{N^\prime })$$\end{document}, such that L=F(L1,⋯,LN′)\documentclass[12pt]{minimal}
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\begin{document}$$L=F(L_1,\dotsc ,L_{N^\prime })$$\end{document}. Several examples are dealt with explicitly.
