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The famous apse mosaic of San Clemente in Rome has been thoroughly studied, but it is so rich that it still has the capacity to surprise. This article focuses on the inscription below the apse and points out that one word is standardly misread: the word is not CRUX (‘Cross’), but CRUS. This turns out to have a highly relevant sense. The article explores the implications of this word and of the inscription more widely.1
A set of vertices in a graph is a Hamiltonian subset if it induces a subgraph containing a Hamiltonian cycle. Kim, Liu, Sharifzadeh, and Staden proved that for large $d$, among all graphs with minimum degree $d$, $K_{d+1}$ minimises the number of Hamiltonian subsets. We prove a near optimal lower bound that takes also the order and the structure of a graph into account. For many natural graph classes, it provides a much better bound than the extremal one ($\approx 2^{d+1}$). Among others, our bound implies that an $n$-vertex $C_4$-free graph with minimum degree $d$ contains at least $n2^{d^{2-o(1)}}$ Hamiltonian subsets.