# Is MathieuC for moderately large imaginary arguments broken?

Bug introduced in 3.0 and persisting through 12.0 (reported as CASE:3208982)

I'm trying to plot MathieuC[-3,0.3,I x] for $$x\in[0,10]$$, and here's what I get even with arbitrary precision arithmetic (here I use ListPlot and Table instead of Plot to make computation faster and use a regular grid):

$MaxExtraPrecision=200; ListPlot[Table[{z,N[Re@MathieuC[-3,3/10,I z],50]},{z,0,7,1/100}], PlotRange->{-0.6,0.6}, Joined->True]  So, starting from about z==3.3, the results are unreliable at all. Comparing machine precision and arbitrary precision computations with N[], I get this: N[Re@MathieuC[-3, 3/10, 5 I]] N[Re@MathieuC[-3, 3/10, 5 I], 50]  -1.26013246174486*10^28 -1.2601324617438073657004964476674284869363635740962*10^28 So, it seems not even lack of working precision — looks like the algorithm is broken. To make sure I'm not misunderstanding the supposed behavior of Mathieu functions, I've also checked it by solving Mathieu equation numerically: With[{a = -3, q = 0.3}, sol = NDSolveValue[{ -w''[z] - 2 q Cosh[2 z] w[z] == -a w[z], w[0] == Re@MathieuC[a, q, 0], w'[0] == 0 }, w, {z, 0, 7}, MaxSteps -> 10^6] ];$MaxExtraPrecision=200;
Show[
Plot[sol[z],{z,0,7}, PlotStyle->Darker@Green, PlotPoints->300],
ListPlot[Table[{z,N[Re@MathieuC[-3,3/10,I z],50]},{z,0,7,1/100}],Joined->True],
PlotRange->{-0.6,0.6}]


This shows that the function indeed should look nicer, but apparently MathieuC can't calculate it for even moderately large imaginary arguments. It also appears that Mathematica versions 5 to 12 give exactly the same results (sometimes with differences in several least significant digits).

Is it a bug, or is it documented somewhere? Are there any workarounds, allowing me to evaluate Mathieu functions not as slowly as via NDSolve, and still not reimplementing them like e.g. here?

• Yes, I think it is broken. But this may be because MathieuC requires exact arguments. Plot feeds it machine precision values and the documentation says it requires exact values under the Properties&Relations section. – Histograms May 14 '15 at 15:24
• @Histograms where did you see such a requirement? I checked built-in docs in MMA 9 and 10 as well as online documentation and didn't see it. The only thing in P&R section is that for some machine-precision arguments the precision is insufficient, and one has to use higher precision in N (or just supply numbers with higher precision, not necessarily exact). In any case, I've given an example of exact arguments and N in the OP, with the same wrong results. – Ruslan May 14 '15 at 17:50
• That's what I was referring to and requirement was a strong word. Perhaps for these choices of the other arguments, the precision problems are amplified, I'm just guessing. – Histograms May 14 '15 at 18:54
• It is a good question. Just to note that it is easier and more convincing to use Plot[Re@MathieuC[-3, 3/10, I z], {z, 0, 10}, PlotRange -> {-0.6, 0.6}, WorkingPrecision -> 50] as the comparison, rather than the one calculated at fixed precision. – Oleksandr R. May 15 '15 at 14:13
• @xzczd what's interesting, when I tried Maple 2015 to see if it has a better implementation (the functions are of the same type there as in MMA: MathieuC and MathieuS), it appeared to fail in exactly the same way. So I suspect Wolfram and Maplesoft used algorithms from the same paper, which appear broken as published, and no one had noticed this. – Ruslan Jun 15 '19 at 15:33