S. D. Isard and A. M. Zwicky,
Three open questions in the theory of one-symbol Smullyan systems,
SIGACT News (1969) 11--14.
They propose the problem: Let if
and let .
Can all numbers or be reduced to 1
by a series of operations f and g?
See [70], p. 148.
if 
and let 
.
Can all numbers 
or 
be reduced to 1
by a series of operations f and g?
See [70], p. 148.