
doi: 10.1007/bf02988317
There has been considerable interest in the methods of deriving congruences from group actions; see \textit{G.-C. Rota} and \textit{B. Sagan} [Eur. J. Comb. 1, 67-76 (1980; Zbl 0453.05008)], or \textit{B. Sagan} [J. Number Theory 20, 210-237 (1985; Zbl 0577.10003)]. The author introduces a different approach to this question. Let \(p\) be a prime and \(H\) a finite set. A wheel system on \(H\) is a family \(\mathcal W\) of subsets of \(H\) (called wheels) with the following three properties: (1) the order of any wheel is a power of \(p\) (a wheel of order \(p^ i\) is called a \(p^ i\)-wheel), (2) distinct wheels of the same order are disjoint, and (3) each \(p^{i+1}\)-wheel is the union of \(p^ i\)-wheels. Wheels of order \(>1\) are called proper. A wheel system \(\mathcal W\) is called complete if \(\mathcal W\) is maximal with respect to the conditions (1), (2) and (3). When one assigns to each proper \(p^ i\)-wheel of a given wheel system \(\mathcal W\) a cyclic ordering of the \(p^{i-1}\)-wheels it contains, \(\mathcal W\) gets an orientation. A spinning is such a permutation of the elements of a given proper wheel that takes its subwheels and preserves its orientation. The subgroup generated by all spinnings of a complete wheel system is the symmetric group \({\mathcal S}_ H\). After proving numerous interesting technical lemmas the author gives some number theoretic applications of his approach. So, he proves (Lemma 3.5): Let \(a= b_ 1+ \cdots+ b_ k\) and suppose \((a,p)= (a,b_ 1,\dots,b_ k)=1\). Then \[ \begin{pmatrix} ap^ n\\ b_ 1 p^ n,\ldots,b_ k p^ n\end{pmatrix}\equiv 0\pmod a. \] Also, he proves the following result (Theorem 5.1): Let \(I(m)\) denote the number of idempotent maps \({\mathbf m}\to {\mathbf m}\). If \(p>2\), then \[ I(p^ n)\equiv 1+ p^{n-1} \left[(\mu-n) p(p-1)+sp+\sum^{p-1}_{k=1} {p\choose k} \sum^ s_{j=0} k^{p+ j-k}{s\choose j}\right]\pmod{p^{n+1}}, \] where \(s=(p-1)(n-1)\) and \(\mu\) denotes the number of proper wheels of a wheel system on \(p^ n\) elements. Concerning some divisibility properties of the Stirling numbers of second kind he proves (Theorem 5.2): Let \(P(m,k)\) denote the number of surjections \({\mathbf m}\to {\mathbf k}\). Then \[ P(p^ n,k)\equiv P(p^{n-1},k)+ p^{n-1} \bigl[P(p+(n-1)(p-1),k)-P(1+ (n-1)(p-1),k)\bigr]\pmod{p^{n+1}}. \]
Permutations, words, matrices, \(p\)-subgroups, Exact enumeration problems, generating functions, Bell and Stirling numbers, Group actions on designs, etc., deriving congruences from group action, Congruences in many variables, Stirling numbers, Subgroups of symmetric groups, symmetric group, Other combinatorial number theory, wheel system, Congruences; primitive roots; residue systems, Combinatorial identities, bijective combinatorics, spinning
Permutations, words, matrices, \(p\)-subgroups, Exact enumeration problems, generating functions, Bell and Stirling numbers, Group actions on designs, etc., deriving congruences from group action, Congruences in many variables, Stirling numbers, Subgroups of symmetric groups, symmetric group, Other combinatorial number theory, wheel system, Congruences; primitive roots; residue systems, Combinatorial identities, bijective combinatorics, spinning
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