I am reading Fourier's "Analytical Theory of Heat" and I am not certain what the meaning of a number preceded by an L (as if it were a bracket). I am guessing that it is the factorial operation - from the context in the picture below, the equation appears to be a Taylor series.
Does anyone know for a certainty what it means?
Old way of writing factorial. See A History of Mathematical Notation
${\displaystyle \vert \!{\underline {\,n}}} = \prod\limits_{k=1}^n k$