# 37 analysis/irrational.stamp.p

## Description

This article is from the Puzzles FAQ,
by Chris Cole chris@questrel.questrel.com and Matthew Daly
mwdaly@pobox.com with numerous contributions by others.

# 37 analysis/irrational.stamp.p

You have an ink stamp which is so amazingly precise that, when inked

and pressed down on the plane, it makes every circle of irrational

radius (centered at the center of the stamp) black.

Question: Can one use the stamp three times and make every point

in the plane black? [assume plane was white to begin with, and

ignore the fact that no such stamp is physically possible]

analysis/irrational.stamp.s

Yes. Center the stamp at (0,0), (1,0), and (0,pi).

Suppose there is a point (x,y) which is not covered.

Then there are rational numbers a,b,c satisfying the following equations:

(1) x^2 + y^2 = a
(2) (x-1)^2 + y^2 = b
(3) x^2 + (y-pi)^2 = c

Subtract (2) from (1) and solve for x. Thus x is rational.

From equation (2), y is algebraic. But equation (3) implies

that y is transcendental, contradiction.

Continue to: