The name is absent

Rectilinear Drawing         53

density for the drawing, and F_e is the similar density cor-
responding to the erasure (so that F_i≥F_e everywhere), then
the final drawing clearly corresponds to the density function
F = F_d-F_e≥0.

A second variant of our problem is obtained if we allow
only a single uniform erasure all over the plane, i.e., modify
F≥k>0 to F — k, where F is the density function for the
given drawing and the constant k corresponds to the uniform
erasure in question.

For our later purposes it is convenient to select a certain
point O in the plane as center for a system of rectangular
coordinates x, y and of related polar coordinates r, θ. Here r
is regarded as positive or negative while θ is an angular
coordinate of period 2τr so that (r, θ) and (— r, θ+π) will

denote the same point. Thus we may regard the density F
as a function of r and θ of period 2π: F = F(r, θ), with
F( — r, 0+7r) = F{r_e θ~). Similarly we may specify a line I

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