
MarcumQ function
Posted:
May 1, 2013 9:40 PM


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

