[Rd] Noncentral dt() with tiny 'x' values (PR#8874)

[meredith at easynet.co.uk]

Full_Name: Mike Meredith
Version: 2.3.0
OS: WinXP SP2
Submission from: (NULL) (210.195.228.29)



Using dt() with a non-centrality parameter and near-zero values for 'x' results
in erratic output. Try this:

tst <- c(1e-12, 1e-13, 1e-14, 1e-15, 1e-16, 1e-17, 0)
dt(tst,16,1)

I get:  0.2381019 0.2385462 0.2296557 0.1851817 0.6288373 3.8163916 (!!)
0.2382217

The 0.238 values are okay, the others nonsense, and they cause confusing spikes
on plots of dt() vs 'x' if 'x' happens to include tiny values. (Other values of
df and ncp also malfunction, but not all give results out by an order of
magnitude!)

I'm using the work-around dt(round(x,10),...), but dt() should really take care
of this itself.

Regards,  Mike.