L-infinity

In mathematics, formula_1, the (real or complex) vector space of bounded sequences with the supremum norm, and formula_2, the vector space of essentially bounded measurable functions with the essential supremum norm, are two closely related Banach spaces. In fact the former is a special case of the latter. As a Banach space they are the continuous dual of the Banach spaces formula_3 of absolutely summable sequences, and formula_4 of absolutely integrable measurable functions (if the measure space fulfills the conditions of being localizable and therefore semifinite). Pointwise multiplication gives them the structure of a Banach algebra, and in fact they are the standard examples of abelian Von Neumann algebras.
Sequence space.
The vector space formula_1 is a sequence space whose elements are the bounded sequences. The vector space operations, addition and scalar multiplication, are applied coordinate by coordinate. With respect to the norm formula_6, formula_7 is a standard example of a Banach space. In fact, formula_1 can be considered as the formula_9 space with the largest formula_10. Moreover, every formula_11 defines a continuous functional on the space formula_12 of absolutely summable sequences by component-wise multiplication and summing:
By evaluating on formula_14 we see that every continuous linear functional on formula_12 arises in this way. i.e. 
Not every continuous linear functional on formula_1 arises from an absolutely summable series however: formula_12, and hence formula_1 is not a reflexive Banach space.
Function space.
L∞ is a function space. Its elements are the "essentially bounded measurable functions". More precisely, L∞ is defined based on an underlying measure space, . Start with the set of all measurable functions from to which are essentially bounded, i.e. bounded up to a set of measure zero. Two such functions are identified if they are equal almost everywhere. Denote the resulting set by .
For a function in this set, its essential supremum serves as an appropriate norm:
See Lp space for more details.
The sequence space is a special case of the function space: formula_21 where the natural numbers are equipped with the counting measure.
Applications.
One application of ℓ∞ and L∞ is in economies with infinitely many commodities. In simple economic models, it is common to assume that there is only a finite number of different commodities, e.g. houses, fruits, cars, etc., so every bundle can be represented by a finite vector, and the consumption set is a vector space with a finite dimension. But in reality, the number of different commodities may be infinite. For example, a "house" is not a single commodity type since the value of a house depends on its location. So the number of different commodities is the number of different locations, which may be considered infinite. In this case, the consumption set is naturally represented by L∞.