# Fourth order obstacle problem

Having solved the obstacle problem for a string, let us turn to a more difficult one, in which an elastic string is replaced with an elastic rod (or plate, if we are looking at a cross-section). Elastic rods resist bending much in the same way that strings don’t. This can be modeled by minimizing the bending energy $\displaystyle B(u) = \frac12 \int_{-2}^2 u''(x)^2 \,dx$

subject to the boundary conditions ${u(-2)=0=u(2)}$, ${u'(-2)=0=u'(2)}$, and the same obstacle as before: ${u(x)\le -\sqrt{1-x^2}}$. The boundary conditions for ${u'}$ mean that the rod is clamped on both ends.

As before, the obstacle permits one-sided variations ${u+\varphi}$ with ${ \varphi\le 0}$ smooth and compactly supported. The linear term of ${B(u+\varphi)}$ is ${\int u''\varphi''}$, which after double integration by parts becomes ${\int u^{(4)} \varphi}$. Since the minimizer satisfies ${E(u+\varphi)-E(u)\ge 0}$, the conclusion is ${\int u^{(4)} \varphi \ge 0 }$ whenever ${\varphi\le 0}$. Therefore, ${u^{(4)}\le 0}$ everywhere, at least in the sense of distributions. In the parts where the rod does not touch the obstacle, we can do variation of either sign and obtain ${u^{(4)}=0}$; that is, ${u}$ is a cubic polynomial there.

So far everything looks similar to the previous post. But the fourth derivative of the obstacle function ${-\sqrt{1-x^2}}$ is ${3(4x^2+1)/(1-x^2)^{7/2}}$, which is positive. Since the minimizer ${u}$ must satisfy ${u^{(4)}\le 0}$, it cannot assume the shape of the obstacle. The contact can happen only at isolated points.

Therefore, ${u}$ is a cubic spline with knots at the contact points and at ${\pm 2}$. The distributional derivative ${u^{(4)}}$ consists of negative point masses placed at the contact points. Integrating twice, we find that ${u''}$ is a piecewise affine concave function; in particular it is continuous. The minimizer will be ${C^2}$-smooth in ${(-2,2)}$.

How many contact points are there? If only one, then by symmetry it must be at ${x=0}$, and the only three-knot cubic spline that satisfies the boundary conditions and passes through ${(0,-1)}$ with zero derivative is ${(-1/4)(1+|x|)(2-|x|)^2}$. But it does not stay below the obstacle:

With a smaller circle, or a longer bar, the one-contact (three-knot) spline would work. For example, on ${[-3,3]}$:

But with our parameters we look for two contact points. By symmetry, the middle piece of the spline must be of the form ${q(x)=cx^2+d}$. The other two will be ${p(x)=(ax+b)(2-x)^2}$ and ${p(-x)}$, also by symmetry and to satisfy the boundary conditions at ${\pm 2}$. At the positive knot ${x_0}$ the following must hold: $\displaystyle p(x_0)=q(x_0)=-\sqrt{1-x_0^2}, \quad p'(x_0)=q'(x_0)=\frac{-x_0}{\sqrt{1-x_0^2}}, \quad p''(x_0)=q''(x_0)$

where the last condition comes from the fact that ${u''}$ is concave and therefore continuous. With five equations and five unknowns, Maple finds solutions in closed form. One of them has ${x_0=0}$, ${a=b=-1/4}$ as above, and is not what we want. The other has ${x_0=(\sqrt{10}-2)/3\approx 0.3874}$ and coefficients such as ${a=-\frac{1}{3645}\sqrt{505875+164940\sqrt{10}}}$. Ugly, but it works:

This time, the bar does stay below the obstacle, touching it only at two points. The amount by which it comes off the obstacle in the middle is very small. Here is the difference ${u(x)-(-\sqrt{1-x^2})}$:

And this is the second derivative ${u''}$.