(I'm not sure about the adjective “harmonic” — maybe it was another one.)
Some years ago I read about harmonic sine function (or sine harmonic one) — probably in some physics textbook — where it was written that it's so common in (some part of physics) that it has its own name (harmonic sine?) and abbreviation.
I'm not able to remember the title of this book, not to find it in Internet and other sources.
I suspect that it was the function $\sin\frac1x$, or the function $x\sin\frac1x$, but I'm not sure.
What is that function, and what are its name and abbreviation?