We extend our previous study of quaternionic analysis based on representation theory to the case of split quaternions H R . The special role of the unit sphere in the classical quaternions H – identified with the group SU ( 2 ) – is now played by the group SL ( 2 , R ) realized by the unit quaternions in H R . As in the previous work, we use an analogue of the Cayley transform to relate the analysis on SL ( 2 , R ) to the analysis on the imaginary Lobachevski space SL ( 2 , C ) / SL ( 2 , R ) identified with the one-sheeted hyperboloid in the Minkowski space M . We study the counterparts of Cauchy–Fueter and Poisson formulas on H R and M and show that they solve the problem of separation of the discrete and continuous series. The continuous series component on H R gives rise to the minimal representation of the conformal group SL ( 4 , R ) , while the discrete series on M provides its K-types realized in a natural polynomial basis. We also obtain a surprising formula for the Plancherel measure of SL ( 2 , R ) in terms of the Poisson-type integral on the split quaternions H R . Finally, we show that the massless singular functions of four-dimensional quantum field theory are nothing but the kernels of projectors onto the discrete and continuous series on the imaginary Lobachevski space SL ( 2 , C ) / SL ( 2 , R ) . Our results once again reveal the central role of the Minkowski space in quaternionic and split quaternionic analysis as well as a deep connection between split quaternionic analysis and the four-dimensional quantum field theory.
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