The question in the title sounds like a trick question, due to the monotonicity property that indifference curves have, such that for two goods x and y, strong monotonicity implies y > x.

Possible that you could answer the question by stipulating the area under a curve?

  • 3
    $\begingroup$ You should specify every one of your assumptions/conditions more clearly. In particular, do you require strong monotonicity? $\endgroup$ – Kenny LJ Jan 13 '20 at 2:10

$$u(x, y) = 2$$ is a simple example of a continuous utility function with thick indifference curve.

\begin{eqnarray*} u(x, y)= \begin{cases} x & x < 1 \\ 1 & 1 \leq x < 2 \\ x - 1 & x \geq 2 \end{cases} \end{eqnarray*}

is another continuous utility function with thick indifference curve for $u=1$.


How about a utility function that only enjoys discrete quantities. This takes discrete values but allows for continuous inputs.

$$U(x,y) = (floor(x))^{\alpha} \cdot (floor(y))^{1-\alpha}$$

enter image description here

    import numpy as np
    import seaborn as sns
    import pandas as pd
    import matplotlib.pyplot as plt

    x = np.arange(0, 10, 0.01)
    y = np.arange(0, 10, 0.01)
    alpha = 0.7

    xv, yv = np.meshgrid(x, y)

    def U(x1,y1, alpha1):
        return(np.floor(x1)**alpha1 * np.floor(y1)**(1-alpha1))

    uv = U(xv,yv, alpha)

    fig, ax = plt.subplots()
    CS = ax.contourf(xv, yv,uv, levels=np.unique(uv))

If it must be strongly monotonic and differentiable, then no, I believe this is impossible. An agent's preferences are said to be strongly monotonic if, given a consumption bundle $x$, the agent prefers all consumption bundles $y$ that have more of at least one good, and not less in any other good. That is $y\geq x$ and $y\neq x$ imply $y\succ x$. If this is a continuous and differentiable utility function then this means: $$ \frac{\partial U}{\partial X_{i}} > 0 \: \forall i$$ This implies that the indifference curves cannot be fat. Consider being on a thin indifference curve and, holding fixed x, increasing y by an infinitesimal amount. This will increase utility by $\frac{\partial U}{\partial y}$, which we just stipulated is positive. So you must be moving onto a higher indifference curve and therefore it is not "fat" in the y direction. Similarly, taking an infinitesimal step in the x direction holding fixed y increase utility by $\frac{\partial U}{\partial x}$, which we also just stipulated is positive. So this utility function cannot have fat indifference curves.

I am unsure if this also is true for non-differentiable utility functions.

  • $\begingroup$ But this again violates (strong) monotonicity. $\endgroup$ – Kenny LJ Feb 15 '20 at 4:28

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