See: Forest Ray Moulton, An Introduction to Celestial Mechanics, 2nd ed. (first published by the Macmillan Company in 1914; reprinted in 1970 by Dover Publications, Inc., Mineola, New York), page 431.
וריאציית הפרמטר, באתר MathWorld (באנגלית) Forest Ray Moulton, An Introduction to Celestial Mechanics, 2nd ed. (first published by the Macmillan Company in 1914; reprinted in 1970 by Dover Publications, Inc., Mineola, New York), page 431.