multivariat hypergeometric function 1F1(1/2,p/2, kappa)
reference: G. HILLIER, R. KAN, and X. WANG: Computationally Efficient Recursions for Top-Order Invariant Polynomials with Applications in Econometric Theory, 25(1), 211-242, 2009. http://www.rotman.utoronto.ca/~kan/papers/zonal5.pdf with many thanks to Raymond Kan