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In page Partial linear space:

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Let S = ( P , B , I ) {\displaystyle S=({\mathcal {P}},{\mathcal {B}},{\textbf {I}})} an incidence structure, for which the elements of P {\displaystyle {\mathcal {P}}} are called points and the elements of B {\displaystyle {\mathcal {B}}} are called lines. S is a partial linear space, if the following axioms hold: