Thanks - but unfortunately I ran into further problems trying these suggestions

shows several discontinuities in the graph (like the ones being caused by the problem with Phiinverse(1-1/2^a)), and goes completely haywire for the larger values of a.

When I tried adding numeric to the definition of Phiinverse and attempted the above plot again, I got:

Error, (in Statistics:-Quantile) invalid argument: probability must evaluate to a floating point value when type numeric is specified, received 1/2^a

Thanks - but unfortunately I ran into further problems trying these suggestions

shows several discontinuities in the graph (like the ones being caused by the problem with Phiinverse(1-1/2^a)), and goes completely haywire for the larger values of a.

When I tried adding numeric to the definition of Phiinverse and attempted the above plot again, I got:

Error, (in Statistics:-Quantile) invalid argument: probability must evaluate to a floating point value when type numeric is specified, received 1/2^a