Bug: Ci(x) and Chi(x) for negative values

[SteenSchmidt]

Hi,

There are three problematic math functions in the Mathematics -> Elementary & Special Functions -> Exponential Integrals palette:

1) 'Cosine Integral.vi' and 'Hyperbolic Cosine Integral.vi' both return real values for negative input values. Neither function is defined in the real domain for negative input values, so should return NaN for inputs <0.

2) 'Dilogarithm.vi' is really evaluating Spence's Integral (as its icon says). The dilogarithm function Li2(x) and spence's integral isn't the same function, so it can't be both. Spence's Integral F(x)=Li2(1-x). The solution would be to a) Make the 'Dilogarithm.vi' really evaluate the dilogarithm function (and fix its icon), and b) To make an additional 'Spence's Integral.vi' that does what 'Dilogarithm.vi' does today.

Cheers,

Steen

[ShazilM]

Hello SteenSchmidt, 

Thank you for your valuable input. I will create a bug report for this and raise it with R&D. 

I have tested everything you mentioned below, but just wanted to verify the 'Hyperbolic Cosine Integral.vi' that you mentioned. 

This correctly returns a value of 74.2099 when the input is -5. (i.e.: cosh(-5) = 74.2099). Is there a crucial Mathematical fundamental that I am missing here?

Regards,

[SteenSchmidt]

Hi Shazil,

It's not the Cosh(z) function that there are problems with, it's the Hyperbolic Cosine Integral:

 

Chi(-5) should return a complex number (20.09+i*pi actually), but the LabVIEW Chi function returns a real number which is only the real part of the proper result. Since the LabVIEW Chi function can't return a complex result, the next best thing would be to make it return NaN instead I think. The same with the Cosine Integral (Ci(z)). The ordinary trig funtions Cos and Cosh works just fine as far as I know.

/Steen

[ShazilM]

Hi Steen, 

Thanks for the description. Im glad this was clarifed. 

Regards,

[ShazilM]

Hello Steen, 

Just to let you know these bugs have been reported. I would like to thank you once again for pointing these out. 

Best Regards,

[SteenSchmidt]

Thanks Shazil,

Do you have a CAR for reference?

/Steen

[ShazilM]

Yes. You can track the CAR using the following reference: 512845

Thanks,