How far back should one go in citing classic works? I once jokingly included in a thesis:


  The equations of motion [1] for a rigid body can be reformulated as…
  
  [1] I. Newton, Philosophiæ Naturalis Principia Mathematica, 1687.


While citing Newton's work for his equations of motion might be over the top, there is a valid underlying question: what criteria can one use to know when a work is “such a classic” that it doesn't warrant citation anymore.

I'll give example in my field:


Ewald summation is a neat trick, and very widely used by people who perform molecular simulation of charged species. It was initially published by Paul Ewald in 1921.
Density functional theory in the Kohn-Sham formalism, published in 1964 (Hohenberg &amp; Kohn) and 1965 (Kohn &amp; Sham).
Nosé–Hoover thermostat, 1984
The ultimate question is what would the average well-informed bachelor's or master's degree-holder in your field be expected to know.

Something that comes up in an undergraduate textbook, or is a commonly known and easily demonstrated fact (for example, the definition of the error function), probably doesn't need to be cited, because everyone in your field would be expected to know it. On the other hand, if you're publishing in a "general interest" journal, then you might want to assume a lower "base" of knowledge, and cite a commonly available source.

However, if it's something that's a very specific tool or fact (or a derivation, etc.), then it probably merits a citation in any case.