In mathematics, a locally integrable function (sometimes also called locally summable function) is a function which is integrable (so its integral is finite) on every compact subset of its domain of definition. The importance of such functions lies in the fact that their function space is similar to spaces, but its members are not required to satisfy any growth restriction on their behavior at the boundary of their domain (at infinity if the domain is unbounded): in other words, locally integrable functions can grow arbitrarily fast at the domain boundary, but are still manageable in a way similar to ordinary integrable functions. Definition Standard definition . Let be an open set in the Euclidean space and be a Lebesgue measurable function. If on is such that i.e. its Lebesgue integral is finite on all compact subsets of , then is called locally integrable. The set of all such functions is denoted by : where denotes the restriction of to the set . The classical definition of a locally integrable function involves only measure theoretic and topological concepts and can be carried over abstract to complex-valued functions on a topological measure space : however, since the most common application of such functions is to distribution theory on Euclidean spaces, all the definitions in this and the following sections deal explicitly only with this important case. An alternative definition . Let be an open set in the Euclidean space . Then a function such that for each test function is called locally integrable, and the set of such functions is denoted by . Here denotes the set of all infinitely differentiable functions with compact support contained in . This definition has its roots in the approach to measure and integration theory based on the concept of continuous linear functional on a topological vector space, developed by the Nicolas Bourbaki school: it is also the one adopted by and by . This "distribution theoretic" definition is equivalent to the standard one, as the following lemma proves: . A given function is locally integrable according to if and only if it is locally integrable according to , i.e. Proof of If part: Let be a test function. It is bounded by its supremum norm , measurable, and has a compact support, let's call it . Hence by . Only if part: Let be a compact subset of the open set . We will first