
Re: MarcumQ function
Posted:
May 3, 2013 3:50 AM


It's a precision issue. Specify a working precision rather than using machine precision; however, it slows down the computations.
Plot[{ MarcumQ[100, 10, x], 1  MarcumQ[100, 10, 0, x]}, {x, 0, 30}, WorkingPrecision > 20]
Plot[{ MarcumQ[51, 10, x], 1  MarcumQ[51, 10, 0, x]}, {x, 0, 30}, WorkingPrecision > 20]
Bob Hanlon
On Wed, May 1, 2013 at 9:43 PM, Donagh Horgan <donagh.horgan@gmail.com>wrote:
> Hello, > > I recently upgraded to Mathematica 9, and I've noticed a problem with the > MarcumQ function. The following command should illustrate it: > > Plot[{MarcumQ[100, 10, x], 1  MarcumQ[100, 10, 0, x]}, {x, 0, 30}] > > While the two functions are mathematically equivalent, the second > incorrectly evaluates to zero between (approximately) x = 14 and x = 17. > > The problem only seems to occur when the first parameter is large. For > example, the command > > Plot[{MarcumQ[50, 10, x], 1  MarcumQ[50, 10, 0, x]}, {x, 0, 30}] > > evaluates correctly, while the command > > Plot[{MarcumQ[51, 10, x], 1  MarcumQ[51, 10, 0, x]}, {x, 0, 30}] > > does not. > > I've tried wrapping the functions with the N command, e.g. N[f, 100], but > the same problem occurs. I've also tried increasing $MaxExtraPrecision and > $WorkingPrecision using Block, but with no luck. > > I've been testing my old code since I upgraded from Mathematica 8 a few > days ago, so the error appears (to me, at least) to be new. > > All suggestions greatly appreciated! > > Best regards, > Donagh Horgan > >

