There is a footnote on Goldstein's Classical Mechanics (2nd ed., page 15) which says the following:
In principle, an integrating factor can always be found for a first-order differential equation of constraint in systems involving only two coordinates(cf. Kaplan, Advanced Calculus, 2d ed.,p.531) and such constraints are therefore holonomic. A familiar example is the two-dimensional motion of a circle rolling on an inclined plane.
the book of Advanced calculus second edition is too old to find. but I got 5th edition of this book. I just wonder what is exactly said on the page 531 or which page is the same part on Advanced calculus 5th edition.
