Intro

Challenges

## The Kiss Precise

For pairs of lips to kiss maybe

Involves no trigonometry.

'Tis not so when four circles kiss

Each one the other three.

To bring this off the four must be

As three in one or one in three.

If one in three, beyond a doubt

Each gets three kisses from without.

If three in one, then is that one

Thrice kissed internally.

Four circles to the kissing come

The smaller are the benter.

The bend is just the inverse of

The distance from the center.

Though their intrigue left Euclid dumb,

There's now no need for rule of thumb.

Since zero bend's a dead straight line,

And concave bends have minus sign,

The sum of the squares of all four bends

Is half the square of their sum.

To spy out spherical affairs

An oscular surveyor

Might find the task laborious,

The sphere is much the gayer,

And now besides the pair of pairs

A fifth sphere in the kissing shares.

Yet, signs and zero as before,

For each to kiss the other four,

The square of the sum of all five bends

Is thrice the sum of their squares.

## The Complex Kiss Precise

Yet more is true: if all four discs

Are sited in the complex plane,

Then centers over radii

Obey the self-same rule again.Wilks, Mallows and Laguna (2001)

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