# Why does N[Re@f] give complex result?

Consider this code:

BlochΚ[κ_, V0_, z_] :=
MathieuC[MathieuCharacteristicA[κ, 2 V0], 2 V0,  z/2] +
Sign[κ] I MathieuS[MathieuCharacteristicB[κ, 2 V0],
2 V0, z/2]

Block[{$MaxExtraPrecision = 500, ϵ = 10^-10}, N[Re@BlochΚ[-2 + ϵ, -1, -10]]] Block[{$MaxExtraPrecision = 1000, ϵ = 10^-20}, N[Re@BlochΚ[-2 + ϵ, -1, -10]]]
Block[{$MaxExtraPrecision = 500, ϵ = 10^-10}, N[Re@BlochΚ[-2 + ϵ, -1, -10]]]  On a fresh kernel I get (* -0.484175 -0.993753 -1.38778*10^-16+6.17104*10^-9 I *)  Why I get complex number at the third time even I used Re explicitly? And why the results are different for the first and third time? Did I made a stupid mistake or what? - Maybe related stackoverflow.com/questions/7798435/… – xslittlegrass Aug 28 at 3:39 ## 2 Answers Update: I think this is a numeric precision problem rather than a matter of the behavior of Re. I don't know if I should leave my original answer below for reference or remove it. Consider: expr = MathieuC[MathieuCharacteristicA[-(19999999999/10000000000), -2], -2, 5]; N[expr] N[expr, 15] SetPrecision[expr, 15]  -9.85323*10^-16 + 3.39211*10^-8 I -0.484175231115992 -0.4841752311160  Only the machine precision calculation returns a complex value. I believe that puts this problem in the same class as: Sorry for the earlier misdirection. Note: I believe $MaxExtraPrecision has no effect upon a machine precision calculations.

Intending to further illuminate Junho Lee's answer we may consider how Re handles symbolic expressions:

Re[a + b I]

-Im[b] + Re[a]


It performs this replacement whether or not a and b have a numeric equivalent. Therefore:

Re[
MathieuC[MathieuCharacteristicA[-(19999999999/10000000000), -2], -2, 5] +
I MathieuS[MathieuCharacteristicB[-(19999999999/10000000000), -2], -2, 5]
]


Becomes:

Re[MathieuC[MathieuCharacteristicA[-(19999999999/10000000000), -2], -2, 5]] -
Im[MathieuS[MathieuCharacteristicB[-(19999999999/10000000000), -2], -2, 5]]


And:

Re[MathieuC[MathieuCharacteristicA[-(19999999999/10000000000), -2], -2, 5]]

MathieuC[MathieuCharacteristicA[-(19999999999/10000000000), -2], -2, 5]

Im[MathieuS[MathieuCharacteristicB[-(19999999999/10000000000), -2], -2, 5]]

0


In some manner Re did its job, nevertheless this symbolic expression has a complex numeric value.

If you want a function that operates only on explicit numbers you might use:

re[x_?NumberQ] := Re[x]


Now re will remain unevaluated until its argument is expressly a number:

re[Pi + 4 I]

re[4 I + π]


However N goes inside as re does not have NHoldFirst etc. therefore:

re[Pi + 4 I] // N

3.14159

-
Did Re do its job wrong? If that's true then it's really scary for me since I used a lot of Mathematica's analytical simplification in my work. I'm wondering whether Re did it correctly but N did it wrong instead. For example N[MathieuC[MathieuCharacteristicA[-(19999999999/10000000000), -2], -2, 5], 10] give me a real number. Maybe this is another expression of this. Mathieu functions have driven me crazy :) –  xslittlegrass Aug 28 at 14:19
@xslittlegrass I am not familiar with these functions (in or out of Mathematica) and I did not check for that, but it seems likely that this is a numeric precision problem after all, which largely invalidates my answer. –  Mr.Wizard Aug 28 at 16:31

I think your problems are made by order of appling Re and N. Re@Bloch is not yet a state before the computation. So you have to apply the computation by Re@Norder.

Block[{$MaxExtraPrecision = 500, ϵ = 10^-10}, Re@N@BlochΚ[-2 + ϵ, -1, -10]] Block[{$MaxExtraPrecision = 1000, ϵ = 10^-20}, Re@N@BlochΚ[-2 + ϵ, -1, -10]]
Block[{$MaxExtraPrecision = 500, ϵ = 10^-10}, Re@N@BlochΚ[-2 + ϵ, -1, -10]]  Block[{$MaxExtraPrecision = 500, ϵ = 10^-10}, Re@BlochΚ[-2 + ϵ, -1, -10]]
Block[{$MaxExtraPrecision = 1000, ϵ = 10^-20}, Re@BlochΚ[-2 + ϵ, -1, -10]] Block[{$MaxExtraPrecision = 500, ϵ = 10^-10}, Re@BlochΚ[-2 + ϵ, -1, -10]]


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