The description of Bessel_K functions in the Sage Cookbook is
confusing about the order of the arguments.
http://sagemath.org/doc/html/const/node96.html
Here's what it says:

>Here's an example using SAGE's interface to pari's special functions.

>sage: pari('2+I').besselk(3)
>0.04559077184075505871203211094 + 0.02891929465820812820828883526*I     # 
>32-bit
>0.045590771840755058712032110938791854704 + 
>0.028919294658208128208288835257608789842*I   # 64-bit
>sage: pari('2').besselk(3)  # random
>0.061510458471742038

>The last command can also be executed using the SAGE command

>sage: bessel_K(3,2)
>0.647385390948634
>sage: bessel_K(3,2,100)
>0.64738539094863415315923557097

But in fact the answer 0.061510458471742038 is correct, not random:

sage: bessel_K(2,3)
0.0615104584717420
sage: bessel_K(2,3,100)
0.061510458471742037656820071453

I believe the confusion resulted because in earlier versions of Sage
the order of arguments to
bessel_K was backwards.  For instance in Sage 2.8.5:

sage: bessel_K(2,3)
0.647385390948634  # WRONG!!!

Cheers,

Peter

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