Let w(x), be the principal
function for the set of all composite nonnegative integers and let Dw denote its
canonical extension to the isols. M. Hassett proved in Comp. M. 26 (1973), as an
application of a very general theorem, that if A is any T-regressive isol then Dw(A) is
a regressive isol that is prime. The present paper contains a number theoretic proof
of the following property: if Y is any infinite multiple-free regressive isol then Dw(Y )
is a regressive isol that is prime.