let G be Group; :: thesis: for N, N1 being normal Subgroup of G st N1 is Subgroup of N holds
ex N2, N3 being strict normal Subgroup of G st
( the carrier of N2 = N1 ~ N & the carrier of N3 = N1 ` N & N3 ~ N c= N2 ~ N )
let N, N1 be normal Subgroup of G; :: thesis: ( N1 is Subgroup of N implies ex N2, N3 being strict normal Subgroup of G st
( the carrier of N2 = N1 ~ N & the carrier of N3 = N1 ` N & N3 ~ N c= N2 ~ N ) )
assume A1: N1 is Subgroup of N ; :: thesis: ex N2, N3 being strict normal Subgroup of G st
( the carrier of N2 = N1 ~ N & the carrier of N3 = N1 ` N & N3 ~ N c= N2 ~ N )
consider N2 being strict normal Subgroup of G such that
A2: the carrier of N2 = N1 ~ N by Th73;
consider N3 being strict normal Subgroup of G such that
A3: the carrier of N3 = N1 ` N by A1, Th74;
N3 is Subgroup of N2 by A2, A3, Th55, GROUP_2:57;
then N3 ~ N c= N2 ~ N by Th57;
hence ex N2, N3 being strict normal Subgroup of G st
( the carrier of N2 = N1 ~ N & the carrier of N3 = N1 ` N & N3 ~ N c= N2 ~ N ) by A2, A3; :: thesis: verum