From "Universal Algebra: Fundamentals and Selected Topics" of Clifford Bergman.
An element $a$ of an algebra $A$ is called a non-generator of $A$ if
for every $X \subseteq A$, $A = Sg(X \cup \{a\})$ implies $A = Sg(X)$.(a) Prove that the set of nongenerators of $A$ forms a subuniverse,
$Frat(A)$ (called the Frattini subuniverse of $A$).(b) Prove that $Frat(A)$ is the intersection of all maximal proper
subuniverses of $A$. (If you wish, you can assume that $A$ is
finite. To do the infinite case, you will need Zornβs lemma.)
My solution for (a)
Suppose $a_1, …, a_n$ to be non-gerators of $A$ and $f$ a fundamental operation of A then:
$X \cup \{f(a_1,…, a_n)\} \subseteq X \cup \{a_1, …, a_n \} \cup \{f(a_1,…, a_n)\}$
It's easy to see that:
$Sg(X \cup \{a_1, …, a_n \} \cup \{f(a_1,…, a_n)\})=Sg(X \cup \{a_1, …, a_n \})=Sg(X)$
Hence $Sg(X \cup \{f(a_1,…, a_n)\}) \subseteq Sg(X)$ and the other verse of inclusion is trivial.
My question
I'm looking for a proof of (b).
Best Answer
Let π = {Mα΅’ : i β I} be the collection of all maximal proper subuniverses of A.
Let Frat(A) be the collection of non-generators of A.
To show: β π = Frat(A)
Frat(A) β β π. Let u be a non-generator of A and show that βi β I, u β Mα΅’ as follows: Fix i. If u does not belong to Mα΅’, then Sg(Mα΅’,u) = A. But then Mα΅’ = Sg(Mα΅’) = A, since u is a non-generator. On the other hand, Mα΅’ is proper. This contradiction proves that u belongs to Mα΅’, and i was arbitrary.
β π β Frat(A). Let u β β π. We prove u is a non-generator. Suppose A = Sg (X βͺ {u}) for some X β A. To see that Sg(X) = A, suppose the contrary; i.e., Sg(X) β A. Then (by Zorn's Lemma) there exists a maximal proper subuniverse M β€ A containing Sg(X). But then u β β π β M implies that M contains both X and u and thus M contains A = Sg (X βͺ {u}), contradicting the fact that M is proper. β