Two Commodities Cannot be Inferior at the Same Time

Oct 13, 2025 by wbrum.org 1 views

Introduction

In the realm of economics, the two-commodity model is a fundamental concept used to analyze the behavior of consumers and producers in a market with two goods. This model is a simplified representation of a real-world market, where consumers make choices between two commodities based on their prices and preferences. In this article, we will delve into the details of the two-commodity model and prove that an+an = 1, where dq, M a = Pili, and η = 9, M; i = 1,2. Additionally, we will demonstrate that two commodities cannot be inferior at the same time.

The Two-Commodity Model: A Mathematical Framework

The two-commodity model is based on the following assumptions:

There are two commodities, denoted as x1 and x2.
The prices of the commodities are denoted as p1 and p2, respectively.
The income of the consumer is denoted as M.
The preferences of the consumer are represented by a utility function, denoted as U(x1, x2).

The utility function is assumed to be twice continuously differentiable and to satisfy the following properties:

U(x1, x2) > 0 for all x1, x2 > 0.
U(x1, x2) is strictly increasing in x1 and x2.
U(x1, x2) is strictly concave in x1 and x2.

The consumer's problem is to maximize the utility function subject to the budget constraint:

p1x1 + p2x2 = M

The Lagrangian function is defined as:

L(x1, x2, λ) = U(x1, x2) - λ(p1x1 + p2x2 - M)

The first-order conditions for a maximum are:

∂L/∂x1 = Ux1 - λp1 = 0 ∂L/∂x2 = Ux2 - λp2 = 0 ∂L/∂λ = p1x1 + p2x2 - M = 0

Proving an+an = 1

To prove that an+an = 1, we need to show that the sum of the marginal utilities of the two commodities is equal to 1.

From the first-order conditions, we have:

Ux1 = λp1 Ux2 = λp2

Taking the ratio of the two equations, we get:

Ux1/Ux2 = p1/p2

Now, we can write the utility function as:

U(x1, x2) = Ux1x1 + Ux2x2

Substituting the expressions for Ux1 and Ux2, we get:

U(x1, x2) = λp1x1 + λp2x2

Simplifying the expression, we get:

U(x1, x2) = λ(p1x1 + p2x2)

Since the utility function is strictly increasing in x1 and x2, we can write:

Ux1 = λp1 > 0 Ux2 = λp2 > 0

Now, we can write the sum of the marginal utilities as:

Ux1 + Ux2 = λp1 + λp2

Substituting the expression for λ, we get:

Ux1 + Ux2 = (p1x1 + p2x2)/M

Since the budget constraint is p1x1 + p2x2 = M, we can write:

Ux1 + Ux2 = M/M = 1

Therefore, we have proved that an+an = 1.

Two Commodities Cannot be Inferior at the Same Time

To show that two commodities cannot be inferior at the same time, we need to demonstrate that the sum of the marginal utilities of the two commodities is always greater than 1.

From the first-order conditions, we have:

Ux1 = λp1 Ux2 = λp2

Taking the ratio of the two equations, we get:

Ux1/Ux2 = p1/p2

Now, we can write the utility function as:

U(x1, x2) = Ux1x1 + Ux2x2

Substituting the expressions for Ux1 and Ux2, we get:

U(x1, x2) = λp1x1 + λp2x2

Simplifying the expression, we get:

U(x1, x2) = λ(p1x1 + p2x2)

Since the utility function is strictly increasing in x1 and x2, we can write:

Ux1 = λp1 > 0 Ux2 = λp2 > 0

Now, we can write the sum of the marginal utilities as:

Ux1 + Ux2 = λp1 + λp2

Substituting the expression for λ, we get:

Ux1 + Ux2 = (p1x1 + p2x2)/M

Since the budget constraint is p1x1 + p2x2 = M, we can write:

Ux1 + Ux2 = M/M = 1

However, this is a contradiction, since we assumed that the sum of the marginal utilities is always greater than 1.

Therefore, we have demonstrated that two commodities cannot be inferior at the same time.

Conclusion

Q: What is the two-commodity model?

A: The two-commodity model is a simplified representation of a real-world market, where consumers make choices between two commodities based on their prices and preferences.

Q: What are the assumptions of the two-commodity model?

A: The two-commodity model is based on the following assumptions:

There are two commodities, denoted as x1 and x2.
The prices of the commodities are denoted as p1 and p2, respectively.
The income of the consumer is denoted as M.
The preferences of the consumer are represented by a utility function, denoted as U(x1, x2).

Q: What is the utility function in the two-commodity model?

A: The utility function is a mathematical representation of the consumer's preferences. It is assumed to be twice continuously differentiable and to satisfy the following properties:

U(x1, x2) > 0 for all x1, x2 > 0.
U(x1, x2) is strictly increasing in x1 and x2.
U(x1, x2) is strictly concave in x1 and x2.

Q: What is the budget constraint in the two-commodity model?

A: The budget constraint is a mathematical representation of the consumer's budget. It is given by:

p1x1 + p2x2 = M

Q: What is the Lagrangian function in the two-commodity model?

A: The Lagrangian function is a mathematical representation of the consumer's problem. It is given by:

L(x1, x2, λ) = U(x1, x2) - λ(p1x1 + p2x2 - M)

Q: What are the first-order conditions for a maximum in the two-commodity model?

A: The first-order conditions for a maximum are:

∂L/∂x1 = Ux1 - λp1 = 0 ∂L/∂x2 = Ux2 - λp2 = 0 ∂L/∂λ = p1x1 + p2x2 - M = 0

Q: How do we prove that an+an = 1 in the two-commodity model?

A: To prove that an+an = 1, we need to show that the sum of the marginal utilities of the two commodities is equal to 1. This can be done by using the first-order conditions and the budget constraint.

Q: Can two commodities be inferior at the same time in the two-commodity model?

A: No, two commodities cannot be inferior at the same time in the two-commodity model. This can be demonstrated by showing that the sum of the marginal utilities of the two commodities is always greater than 1.

Q: What are the implications of the two-commodity model?

A: The two-commodity model has several implications for consumer behavior and market equilibrium. It shows that consumers make choices based on their prices and preferences, and that the sum of the marginal utilities of the two commodities is always greater than 1.

Q: What are the limitations of the two-commodity model?

A: The two-commodity model is a simplified representation of a real-world market, and it has several limitations. It assumes that consumers have a fixed income and that the prices of the commodities are fixed. It also assumes that the consumer's preferences are represented by a utility function, which may not be the case in reality.

Q: How can the two-commodity model be extended to a multi-commodity model?

A: The two-commodity model can be extended to a multi-commodity model by adding more commodities and prices to the model. This can be done by using the same mathematical framework and assumptions as the two-commodity model.

Q: What are the applications of the two-commodity model?

A: The two-commodity model has several applications in economics, including:

Consumer behavior: The two-commodity model can be used to analyze consumer behavior and preferences.
Market equilibrium: The two-commodity model can be used to analyze market equilibrium and the behavior of firms.
Welfare economics: The two-commodity model can be used to analyze welfare economics and the distribution of income.

Q: What are the future directions of research in the two-commodity model?

A: There are several future directions of research in the two-commodity model, including:

Extensions to a multi-commodity model: Researchers can extend the two-commodity model to a multi-commodity model by adding more commodities and prices.
Applications to real-world markets: Researchers can apply the two-commodity model to real-world markets to analyze consumer behavior and market equilibrium.
Development of new mathematical tools: Researchers can develop new mathematical tools and techniques to analyze the two-commodity model and its extensions.