For subnormal subgroups \documentclass[12pt]{minimal}
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\begin{document}$${A{\vartriangleleft}B}$$\end{document} and \documentclass[12pt]{minimal}
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\begin{document}$${C{\vartriangleleft}D}$$\end{document} of a given group G, the factor B/A will be called subnormally down-and-up projective to D/C if there are subnormal subgroups \documentclass[12pt]{minimal}
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\begin{document}$${X{\vartriangleleft}Y}$$\end{document} such that \documentclass[12pt]{minimal}
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\begin{document}$${AY = B, A \cap Y = X, CY = D}$$\end{document} , and \documentclass[12pt]{minimal}
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\begin{document}$${C \cap Y = X}$$\end{document} . Clearly, \documentclass[12pt]{minimal}
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\begin{document}$${B/A \cong D/C}$$\end{document} in this case. As G. Grätzer and J. B. Nation have recently pointed out, the standard proof of the classical Jordan-Hölder theorem yields somewhat more than is widely known; namely, the factors of any two given composition series are the same up to subnormal down-and-up projectivity and a permutation. We prove the uniqueness of this permutation.