A characterization of the intersection of L1-spaces SIGURDUR HELGASON — 1956 Mathematica Scandinavica
Some Remarks on the Exponential Mapping for an Affine Connection. Sigurdur Helgason — 1961 Mathematica Scandinavica
Some problems in the theory of almost periodic functions SIGURDUR HELGASON — 1955 Mathematica Scandinavica
Some results on eigenfunctions on symmetric spaces and eigenspace representations. Sigurdur Helgason — 1977 Mathematica Scandinavica
Remarque sur «R. G. McLenaghan : on the validity of Huygens principle for second order partial differential equations with four independent variables. I» Sigurdur Helgason — 1974 Annales de l'I.H.P. Physique théorique