Combinatorial Intricacies of Labeled Fano Planes
Résumé
Given a seven-element set X = {1, 2, 3, 4, 5, 6, 7}, there are 30 ways to define a Fano plane on it. Let us call a line of such Fano plane, that is to say an unordered triple from X, ordinary or defective according as the sum of two smaller integers from the triple is or is not equal to the remaining one, respectively. A point of the labeled Fano plane is said to be of order s, 0 ≤ s ≤ 3, if there are s defective lines passing through it. With such structural refinement in mind, the 30 Fano planes are shown to fall into eight distinct types. Out of the total of 35 lines, nine ordinary lines are of five different kinds, whereas the remaining 26 defective lines yield as many as ten distinct types. It is shown, in particular, that no labeled Fano plane can have all points of zeroth order, or feature just one point of order two. A connection with prominent configurations in Steiner triple systems is also pointed out.
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