gr.group theory - Finding a subnormal series with specified quotients and end group of specific depth (defect)

This can always be done:

Given nontrivial groups $A_i$ for $0 le i le n$, there exists a group $G$ and a subnormal series $H = H_0 < cdots < H_n = G$ such that $H_i/H_{i-1} cong A_i$ for $0 le i < n$ and such that no shorter subnormal series from $H$ to $G$ exists.

Here is my proof:

We can assume $n > 1$, and we induct on $n$. By the inductive hypothesis, let $W$ be a group with subnormal series $V = V_1 < cdots < V_n$, such that $V_i/V_{i-1} cong A_i$ for $1 le i < n$, and such that there exists no shorter subnormal series for $V$ in $W$. Write $A = A_0$ and let $G$ be the wreath product of $A$ with $W$ corresponding to the action of $W$ on the right cosets in $V$. In other words, $G = BW$ is a semidirect product, where $B triangleleft G$ and $B$ is the direct product of $|W:V|$ copies of $A$. Also, $W$ acts to permute these direct factors of $B$, and this action is permutation isomorphic to the action of $W$ on the cosets of $V$ in $W$. (In fact, we assume that we are given a specific bijection from the set of cosets of $V$ onto the set of direct factors of $B$.)

Now let $C$ be the product of all of the direct factors of $B$ that correspond to nontrivial cosets of $V$, and note that ${bf N}_W(C) = V$. Let $H = H_0$ be the group $CV$, and for $i > 0$, let $H_i = BV_i$. It is easy to see that $H_0 < H_1 < cdots < H_n = G$ is a subnormal series with factors $A_i$ as wanted. We must show that no shorter subnormal series for $H$ exists. Note that the subnormal depth of $H_1$ is exactly $n - 1$. (This can be seen by intersecting a subnornal series for $H_1$ in $G$ with $W$. This yields a subnormal series for $V$ in $W$.)

Suppose $H triangleleft K$. We argue that $BK = BV$. Otherwise,
$BK > BV$, so $BK cap W > V$. But $BK$ normalizes $C$ since $C = B cap H$, and this contradicts the fact that $V$ is the full normalizer of $C$ in $W$. Now if $H = K_0 < K_1 < cdots < K_m = G$ is a subnormal series for $H$, then $H_1 = BV = BK_1 subseteq cdots subseteq BK_m = G$ is a subnormal series for $H_1$ with length at most $m-1$, and thus $m ge n$, as wanted.

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