# Origin of $j$-invariant

It is often asserted that the $$j$$-invariant was first introduced by Felix Klein. Is there any evidence for this claim? What works of Felix Klein do deal with it? What is the origin of the symbol $$j$$ used for it? Where it had first appeared?

Notation: Klein used capital $$J$$, there is a factor 1728 difference with the $$j$$-invariant. (Read more about the origin of this factor.)