add ln note

book/P0C0_notation.qmd

@@ -47,6 +47,8 @@ A function $f$, will either be written as a formal map of domain to codomain, $f
 Given a random variable, $X$, following distribution $\zeta$ (mathematically written $X \sim \zeta$), then $f_X$ denotes the probability density function, and analogously for other distribution defining functions such as the cumulative distribution function, survival function, etc.
 In the survival analysis context (@sec-surv), a subscript "$0$" refers to a "baseline" function, for example, $S_0$ is the baseline survival function.
 
+Finally $\exp$ refers to the exponential function and $\log$ refers to the natural logarithm $\ln(x) = \log_e(x)$.
+
 ## Variables and acronyms
 
 Common variables and acronyms used in the book are given in @tbl-not-var and @tbl-not-acr respectively.