# Leontief utilities

In economics, especially in consumer theory, a Leontief utility function is a function of the form: $u(x_{1},\ldots ,x_{m})=\min \left\{{\frac {x_{1}}{w_{1}}},\ldots ,{\frac {x_{m}}{w_{m}}}\right\}.$ where:
* $m$ is the number of different goods in the economy.
* $x_{i}$ (for $i\in 1,\dots ,m$) is the amount of good $i$ in the bundle.
* $w_{i}$ (for $i\in 1,\dots ,m$) is the weight of good $i$ for the consumer.
This form of utility function was first conceptualized by Wassily Leontief.

## Examples

Leontief utility functions represent complementary goods. For example:
* Suppose $x_{1}$ is the number of left shoes and $x_{2}$ the number of right shoes. A consumer can only use pairs of shoes. Hence, his utility is $\min(x_{1},x_{2})$.
* In a cloud computing environment, there is a large server that runs many different tasks. Suppose a certain type of a task requires 2 CPUs, 3 gigabytes of memory and 4 gigabytes of disk-space to complete. The utility of the user is equal to the number of completed tasks. Hence, it can be represented by: min(x$_{CPU}$/2, x$_{MEM}$/3, x$_{DISK}$/4){\textstyle \min({x_{\mathrm {CPU} } \over 2}, {x_{\mathrm {MEM} } \over 3}, {x_{\mathrm {DISK} } \over 4})}.

## Properties

A consumer with a Leontief utility function has the following properties:
* The preferences are weakly monotone but not strongly monotone: having a larger quantity of a single good does not increase utility, but having a larger quantity of all goods does.
* The preferences are weakly convex, but not strictly convex: a mix of two equivalent bundles may be either equivalent to or better than the original bundles.
* The indifference curves are L-shaped and their corners are determined by the weights. E.g., for the function $\min(x_{1}/2,x_{2}/3)$, the corners of the indifferent curves are at $(2t,3t)$ where $t\in [0,\infty )$.
* The consumer's demand is always to get the goods in constant ratios determined by the weights, i.e. the consumer demands a bundle $(w_{1}t,\ldots ,w_{m}t)$ where $t$ is determined by the income: $t={\text{Income}}/(p_{1}w_{1}+\dots +p_{m}w_{m})$. Since the Marshallian demand function of every good is increasing in income, all goods are normal goods.

## Competitive equilibrium

Since Leontief utilities are not strictly convex, they do not satisfy the requirements of the Arrow–Debreu model for existence of a competitive equilibrium. Indeed, a Leontief economy is not guaranteed to have a competitive equilibrium. There are restricted families of Leontief economies that do have a competitive equilibrium.
There is a reduction from the problem of finding a Nash equilibrium in a bimatrix game to the problem of finding a competitive equilibrium in a Leontief economy. This has several implications:
* It is NP-hard to say whether a particular family of Leontief exchange economies, that is guaranteed to have at least one equilibrium, has more than one equilibrium.
* It is NP-hard to decide whether a Leontief economy has an equilibrium.
Moreover, the Leontief market exchange problem does not have a fully polynomial-time approximation scheme, unless PPAD ⊆ P.
On the other hand, there are algorithms for finding an approximate equilibrium for some special Leontief economies.

## Application

Dominant resource fairness is a common rule for resource allocation in cloud computing systems, which assums that users have Leontief preferences.