[1] J. KOPÁČEK:
The Cauchy problem for linear hyperbolic systems in $L_p$. Comment. Math. Univ. Carolinae 8 (1967), No 3, 458-462.
MR 0222479
[2] L. HÖRMANDER:
Estimates for translation invariant operators in $L_p$ spaces. Acta Math. 104 (1960), 93-140.
MR 0121655
[3] A. P. CALDERÓN:
Lebesgue spaces of differentiable functions and distributions. Proc. of symp. in pure Math. Vol. IV, Part. dif. equations, Amer. Math. Soc. Providence 1961.
MR 0143037
[4] L. A. MURAVEJ: Zadača Koši dlja volnovogo uravnenija v $L_p$ -prostranstvech. Trudy Mat. inst. im. Steklova 103 (1968), 172-180.