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单词 FiniteProjectivePlanesHaveQ2q1PointsAndQ2q1Lines
释义

finite projective planes have q2+q+1 points and q2+q+1 lines


Given a finite projective plane that contains a quadrangle OXYZ (i.e. nothree of these four points are on one line). To prove: the plane hasq2+q+1 points and q2+q+1 lines for some integer q, and there are q+1points on each line and q+1 lines through each point.

Let x and y be the lines OX andOY, which must exist by the axioms. By the assumptionPlanetmathPlanetmath OXYZ is a quadranglethese lines are distinct and Z is not on them. Let there be p points Xi onx other than O, for each of them one line ZXi exists, and is distinct(one lines cannot pass through two Xi unless it is x but that’s not aline through Z). Conversely every line through Z must intersect x in aunique point (two lines intersecting in Z cannot intersect at another point,and Z is not a point on x). So there are p+1 lines through Z (OZ is oneof them). By the same reasoning, using y, there are q+1 lines through Zso p=q. We also found q+1 points (including O) on y and the same numberon x. Intersecting the q+1 lines through Z with XY (on which Z does notlie, the quadrangle again) reveals at least q+1 distinct points there andat most q+1 because for each point there there is a line through it and Z.

The lines not through O intersect x in one of the q points Xi and yin one of the q points Yj. There are q2 possibilities and each of themis a distinct line, because there is only one line through a given Xi andYj. The lines that do pass through O intersect XY in one of the q+1points there, again one line for each such point and vice versa. That’s q+1lines through O and q2 not through O, q2+q+1 in all.

There are q+1 lines through X (to each of the points of y) and q+1lines through Y (to each of the points of x). Intersect the q linesthrough X other than XY with the q lines through Y other than XY, theseq2 intersections are all distinct because for any P there’s only one linePX and one line PY. Note we did not use the line XY. Conversely for any P noton XY there must be some PX and some PY, so there are exactly q2 points noton XY. Add the q+1 points on XY for a total of q2+q+1.

The constructions above already showed q+1 lines through some points (X, Yand Z), by the same games as before that implies for each of them q+1 pointson every line not through that point. We also saw q+1 points on some lines(x, y, XY) which implies for each of them q+1 lines through every pointnot on that line. Such reasoning covers q2 items on first application andrapidly mops up stragglers on repeated application.

Some form of this proof is standard math lore; this version was half remembered and half reconstructed.

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更新时间:2025/5/4 19:51:30