# The Y combinator in graphic lambda calculus and in the chemical concrete machine

A simpler computation than the one with the Ackermann machine concerns the Y combinator.  Seen as a chemical reaction network, it looks like this for graphic lambda calculus. In the figure “A” is any graph in $GRAPH$ which has only one exit arrow, for example a combinator graph.

One might prefer to not use the GLOBAL FAN-OUT move. That’s possible, by passing to the chemical concrete machine formalism. The chemical reaction network is a bit different. (Notice the move PROP+, which is a composite move defined in the post Chemical concrete machine, detailed (VI). )

Lots of interesting things happen, among them we notice the appearance of  stacks of “elementary units from the last post. so in the chemical concrete machine version of the behaviour of the Y combinator, the machine counts the number of times A should be replicated, if known (that’s a kind of lazy evaluation, if evaluation would make sense in this formalism).

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