where 
is the delta function.
See also
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References
von Seggern, D. CRC Standard Curves and Surfaces. Boca Raton, FL: CRC Press, p. 324, 1993.
Referenced on Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Doublet Function." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/DoubletFunction.html