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Topic: MarcumQ function
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Donagh Horgan

Posts: 7
Registered: 4/6/12
MarcumQ function
Posted: May 1, 2013 9:40 PM
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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

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