Veblen, Oswald, 1880-1960, Young, John Wesley, 1879-1932 · Boston, New York [etc.] Ginn and Company · leaf 85
edouble points of each projectivity in the group; and also that if P, Pand Q, Qr are any two pairs of homologous points of a projectivitywe have Q(MPQ, NQP). Moreover, it is clear that any projectivity,of the group is uniquely determined by a pair of homologous elements,and that there exists a… … Example 2. Commutative projectivities. Let M be a point of aline I, and let m, m be any two lines through M distinct from I, butin the same plane with I (fig. 28.) Let S be a given point of m, andlet a projectivity ir1 be defined by another point S1 of m which deter- mines the perspectivities [A] =…
Geometry, Projective
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