The Values of Exponential Polynomials at Algebraic Points. I
Carlos Julio Moreno
Transactions of the American Mathematical Society
Vol. 186, (Dec., 1973), pp. 17-31

http://www.jstor.org/pss/1996549?searchUrl=%2Faction%2FdoBasicSearch%3FQuery%3DExponential%2BPolynomials%26acc%3Doff%26wc%3Don&Search=yes