@article{Bam-Eng-Hal-Jol-1988-1,
    author={Alain Bamberger and Bjorn Engquist and Laurence Halpern and 
           Patrick Joly },
    title={Parabolic Wave Equation Approximations in heterogenous media },
    journal={SIAM Journal on Applied Maths },
    year={1988 },
    volume={28, 1 },
    pages={99--128},
}