Off topic: what is the "Riemann boundary-value problem"

I know that this question is not this groups primary interest, but I
like very much the answers posted here.

So, what is the Riemann boundary-value problem?

I see it mentioned a lot in Chuck Sievert's works on exact solutions to
some transcendental equations (http://www4.ncsu.edu/~ces)

Thanks,

Mirko

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