On Mon Jan 17, 2000, Stephen Silver wrote:
Subject: phoenices
A phoenix is a pattern all cells of which die in every generation, but
which never dies as a whole. All phoenices, oscillators and spaceships
in this post are assumed to be finite.
It's easy to show that a spaceship cannot be a phoenix (see proof
below), so I'm mainly interested in phoenix oscillators. The existence
of p2 phoenix oscillators is well-known, but what about higher periods?
I can prove that there are no p3 examples (see proof below) but I can't
see any obvious reason why there shouldn't be a p4 phoenix, or a p5
phoenix, for example. I modified a copy of lifesrc to look only for
phoenices, but my small scale searches so far haven't revealed any
meaningful partial results for periods greater than 2, let alone a
complete oscillator.
Has anyone else looked at this problem?
Stephen
Here are the promised proofs:
Theorem A. No phoenix can ever extend more than one space outside its
original bounding box.
Proof. Suppose it is about to extend two spaces to the right of the
original bounding box for the first time. Then we have
where the ? cells are inside the original bounding box and the O cells
are just outside it. The O cells must be newborn, so all the ? cells
must have been ON in the previous generation in order to give birth to
the central O cell. In the current generation the ? cells are therefore
OFF. So the central O cell has exactly two neighbours and will survive
into the next generation - a contradiction.
Corollary A1. Every phoenix evolves into a phoenix oscillator.
Corollary A2. No spaceship is a phoenix.
Note. The theorem holds in a number of other cellular automata.
The proof requires only that births not be possible with less than
3 neighbours and that survival occurs with a particular arrangement
of 2 neighbours.
Theorem B. If an oscillator is such that no cell is ON in more than one
phase then it is either a still life or a statorless p2.
Proof. Suppose there is a counterexample. Cells ON in some chosen phase
will be said to be of type Z. Those ON in the previous phase will be called
type Y, those on in the phase before that will be type X, and those of the
phase before that type W. Since the period is at least 3 these types are
all distinct, except that we may have W=Z.
In the following diagrams . marks a cell that is definitely not in the
rotor (and is therefore permanently off), while known rotor cells are marked
Z, Y, X or W according to their type. Cells which are unknown or not
discussed here are marked with a ?.
Consider the leftmost column of the rotor, and in particular the uppermost
rotor cell of this column. We can assume this cell is of type Z. Three of
its neighbours will be of type Y, namely its parents.
Consider the cell to the right of the Z cell (marked x here):
If x is of type Y then it has at least two Y neighbours and so at least
four (to kill it off). It also has three X neighbours (its parents).
Together with its Z neighbour and its non-rotor neighbour this is a total
of nine neighbours. This contradiction shows that x is not of type Y, so
we have the following diagram:
where I have marked in the three X parents of the leftmost Y. But now the
leftmost X has no room for its own W parents.
So the counterexample does not exist.
Corollary B1. There is no p3 phoenix.