Re: MarcumQ function

*To*: mathgroup at smc.vnet.net*Subject*: [mg130698] Re: MarcumQ function*From*: Bob Hanlon <hanlonr357 at gmail.com>*Date*: Fri, 3 May 2013 03:52:29 -0400 (EDT)*Delivered-to*: l-mathgroup@mail-archive0.wolfram.com*Delivered-to*: l-mathgroup@wolfram.com*Delivered-to*: mathgroup-outx@smc.vnet.net*Delivered-to*: mathgroup-newsendx@smc.vnet.net*References*: <20130502014331.5FD146A86@smc.vnet.net>

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 at 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 > >

**References**:**MarcumQ function***From:*Donagh Horgan <donagh.horgan@gmail.com>