MathDB

A ball

K

of radius

r

is touched from the outside by mutually equal balls of radius

R

. Two of these balls are tangent to each other. Moreover, for two balls

K_1

and

K_2

tangent to

K

and tangent to each other there exist two other balls tangent to

K_1,K_2

and also to

K

. How many balls are tangent to

K

? For a given

r

determine

R

Problem Statement