My book says that a function $f:X\to\mathbb{R}$ is derivable at point $\sigma\in X$ if there exists a function $\varphi\_{\sigma}:X\to\mathbb{R}$ which is continuous at $\sigma$ such that $f(x)-f(\sigma)=\varphi\_{\sigma}(x)(x-\sigma)$. My book says that since every function is continuous at a isolated point of its domain, then it only considers the derivatives of points of accumulation of the domain of $f$. So I'm curious to know if all the points in which $f$ have derivatives are points of accumulation of the domain.