1. Allen, C. W.: 1973,Astrophysical Quantities, The Athlone Press, London.
2. Bevington, P. R.: 1969,Data Reduction and Error Analysis for the Physical Sciences, McGraw-Hill Book Co., p. 84.
3. Budding, E.: 1973,Astrophys. Space Sci. 22, 87.
4. Budding, E.: 1974a,Astrophys. Space Sci. 26, 371.
5. Budding, E.: 1974b,Astrophys. Space Sci. 29, 17.