# Locally integrable function

at a point $M$
A function that is integrable in some sense or other in a neighbourhood of $M$. If a real-valued function $f$, defined on the interval $[a,b]$, is the pointwise finite derivative of a function $F$, real-valued and defined on this interval, then $f$ is locally Lebesgue integrable at the points of an open everywhere-dense set on $[a,b]$. In the two-dimensional case (see [2]) there is a real-valued function $f$, defined on the square $[0,1]\times[0,1]$, that is the pointwise finite mixed derivative in either order $\partial^2F/\partial x\partial y=\partial^2F/\partial y\partial x=f(x,y)$ and that is not locally Lebesgue integrable at any point of the square.