# Problem with the function N with second argument

I was asked to look at a complicated definite integral that could be integrated analytically, but numerically did not behave as expected. After a lot of simplifications I arrived at the following:

In:= int= Integrate[x BesselJ[0, 1/x], {x, 0, 1/2000}]

Out= -(1999999/8000000)+EulerGamma/4-
31250 HypergeometricPFQ[{1,1},{2,3,3},-1000000]+(3 Log)/4


When we inspect the plot of the integrand, we see that the absolute value of the integral must be considerably less than $$2\times 10^{-5}$$. We find:

In:= N[int]

Out= -2.04448*10^-12

In:= N[int, 5]

Out= 1.6212


The second result seems to be wrong. The culprit is the hypergeometric function:

In:= N[ HypergeometricPFQ[{1,1},{2,3,3},-1000000]]

Out= 0.0000518798

In:= N[ HypergeometricPFQ[{1,1},{2,3,3},-1000000], 5]

Out= 6.5424*10^-17


I also observed the very frustrating behaviour that I cannot always reproduce the incorrect output shown above. At the moment, Mathematica behaves stable: when I evaluate the above cells once again, I obtain the same output. But not rarely I observed that the results of N with a second argument suddenly were in agreement with the output without a second argument. To see the erroneous behavior again, I had to change the upper limit in the integral a little bit. I am running version 11.3 under Windows 10.

Am I right that the above arbitrary precision result for HypergeometricPFQ is incorrect? Can somebody reproduce the incorrect result?

• NIntegrate[x BesselJ[0, 1/x], {x, 0, 1/2000}, WorkingPrecision -> 32] suggests the arb. prec. result is wrong. and the machine precision result is accurate. (V11.3.0) – Michael E2 Jul 2 '18 at 18:00
• Also suggested by N[HypergeometricPFQ[{1, 1}, {2, 3, 3}, -1000000], 100] – ilian Jul 2 '18 at 19:23
• @Ilian. Your result shows that you arrived at the correct result. My frustration is that sometimes, I do not know when, I get this correct result as well. Did you see the incorrect result? – Fred Simons Jul 3 '18 at 7:06
• What version are you using? In Version 11.3 I get consistent and correct answers for all these cases. – TheDoctor Jul 8 '18 at 2:31
• @TheDoctor, Thanks. It looks like I am the only one getting this incorrect result (V11.3, windows). I reported it to WRI. – Fred Simons Jul 8 '18 at 8:09

Here is a workaround. The integral under consideration, after making a substitution, can be expressed as

$$\int_{2000}^\infty\frac{J_0(u)}{u^3}\mathrm du$$

Through Mellin transform techniques, this can be converted to a Meijer $$G$$ expression:

$$\frac{1}{8}\operatorname{G}_{1,3}^{2,0}\left(\frac{2000}{2},\frac{1}{2}\middle |{{1}\atop{-1,0,-1}}\right)$$

or in Mathematica syntax, MeijerG[{{}, {1}}, {{-1, 0}, {-1}}, 2000/2, 1/2]/8. Using FunctionExpand[] on it confirms the closed form given in the OP.

This seems to work with arbitrary precision, unlike the hypergeometric expression:

N[MeijerG[{{}, {1}}, {{-1, 0}, {-1}}, 2000/2, 1/2]/8, 5]
-2.0445*10^-12


Alternatively, a slight change in the Mellin route yields the equivalent Meijer $$G$$ expression

$$\frac{1}{8}\operatorname{G}_{3,1}^{0,2}\left(\frac{2}{2000},\frac{1}{2}\middle |{{1,2,2}\atop{0}}\right)$$

(in Mathematica form, MeijerG[{{1, 2}, {2}}, {{}, {0}}, 2/2000, 1/2]/8) which is related to the first one via the "flip identity". This, too, seems to also work for arbitrary precision evaluations.

• Just back from a short holiday, I found your answer. That is a wonderful piece of mathematics! I forwarded your answer to the person who asked me to look at the integral. – Fred Simons Oct 1 '18 at 16:46