# For what positive value of $c$ does the equation $\log(x)=cx^4$ have exactly one real root?

For what positive value of $c$ does the equation $\log(x)=cx^4$ have exactly one real root?

I think I should find a way to apply IMV and Rolle's theorem to $f(x) = \log(x) - cx^4$. I think I should first find a range of values for $c$ such that the given equation has a solution and then try to find one that gives only a unique solution. I have thought over it but nothing comes to my mind. Perhaps I'm over-complicating it.

Any ideas on how to proceed are appreciated.

## 5 Answers

If $f(x)=\ln(x)$ and $g(x) = cx^4$ has exactly $1$ solution, then we have a condition of tangency at their common point.

So $$\displaystyle \left[f'(x)\right]_{(x_{1},y_{1})} = \frac{1}{x_{1}}$$ and $$\displaystyle \left[g'(x)\right]_{(x_{1},y_{1})} = 4cx^3_{1}$$

Note at $P(x_{1},y_{1})$ these slopes are equal

So $$\displaystyle \left[f'(x)\right]_{(x_{1},y_{1})} = \left[g'(x)\right]_{(x_{1},y_{1})}\Rightarrow \frac{1}{x_{1}} = 4cx^3_{1}\Rightarrow x^4_{1} = \frac{1}{4c}$$

Now point $P(x_{1},y_{1})$ also lies on $f(x)$ and $g(x)$

So $$\displaystyle \ln(x_{1}) = cx^4_{1} = c\cdot \frac{1}{4c} = \frac{1}{4}$$

So we get $$\displaystyle \ln(x_{1}) = \frac{1}{4}\Rightarrow x_{1} = e^{\frac{1}{4}}$$

So put $\displaystyle x = \frac{1}{4}$ into $\ln(x_{1}) = cx^4_{1}\;,$ and we get $\displaystyle \frac{1}{4} = c\cdot e \displaystyle \Rightarrow c = \frac{1}{4e}$

Note that for all $c\le0\;,$ these two curves intersect each other at exactly one point, but we're only interested in values of $c$ for which $c>0$.

So our final solution is $\displaystyle c = \left\{\frac{1}{4e}\right\}$

• $\ln(x) = 0\Rightarrow x = e^{0} = 1$ and $\displaystyle c= \frac{1}{2e}$ – juantheron Sep 8 '15 at 7:39
• Put $x=1$ in $\ln(x)$ – juantheron Sep 8 '15 at 7:41
• Oops, right. Fortunately the problem has excluded non-positive solutions for $c$. XD – user246836 Sep 8 '15 at 7:43
• Sorry HZ I have edited by solution, You are Right, Ans $\displaystyle = \frac{1}{4e}$ – juantheron Sep 8 '15 at 7:45
• Why must the slopes be equal at the intersection point? I ask since it is not guaranteed that two intersecting curves will have a common tangent line. Intersecting lines is an easy counter example. So why can we assume that the two curves in this problem will share a tangent line? – rosterherik Oct 26 '18 at 22:10

Hint: Let $x = k$ be the unique place at which both curves intersect. Then we also know that both curves have a common tangent line at this point (try sketching a few example curves to see why), so we can equate derivatives. Thus, we must solve the following system of equations (assuming $\log$ refers to the natural log $\ln$): \begin{cases} \ln k = ck^4 \\ \dfrac{1}{k} = 4ck^3 \end{cases}

There will be precisely one real root if and only if $c\leq 0$ or $c={1\over 4e}$. The details are below.

If $c\leq 0$, then the derivative ${1\over x}-4cx^3$ is strictly positive in $(x,\infty)$, and since the function $\log (x)-cx^4$ tends to $-\infty$ as $x\to 0$ (from the right) and tends to $+\infty$ as $x\to\infty$, you get that there is one real root, by continuity, and it is unique, because the function is strictly increasing.

If $c > 0$, the the derivative ${1\over x}-4cx^3$ has precisely one real root, (check this), and the function tends to $-\infty$ at both extremes, so the only way it will have a unique root is when the $x$ axis is tangent to the graph, i.e. when both the function and it's derivative are zero. Solving the equations gives $c={1\over 4e}$.

Added in proof: I did not notice that you originally asked for only the positive cases of $c$. This leaves you only with $c={1\over 4e}$.

Solution in terms of Lambert W function:

\begin{align} \log(x)&=cx^4, \\ x^{-4}\log(x^{-4})&=-4c, \\ \log(x^{-4})&=\mathop{W}(-4c), \\ x&=\exp\left(-\tfrac14\mathop{W}(-4c)\right). \end{align}

For $c>0$ there is only one point $t=-\exp(-1)$, where the two real branches $\mathop{W_0}(t)$ and $\mathop{W_{-1}(t)}$ met, so the answer is $c=\tfrac14\exp(-1)$.

• How did you arrive at the 3rd equation? – user246836 Sep 8 '15 at 9:01
• @H.Z. Hint: $x^{-4}\log(x^{-4})=\log(x^{-4})\exp(\log(x^{-4}))$. – g.kov Sep 8 '15 at 9:47

One may use a slightly geometric argument for this problem. Notice that $$\ln(x)=cx^4$$ when $$f(x)=cx^4-\ln x=0$$. Now, $$f$$ has a vertex, and the vertex occurs where $$f'(x)=4cx^3-\frac{1}{x}=0$$. Thus, we have the system of equations

\begin{align} f(x)&=cx^4-\ln x=0\\ f'(x)&=4cx^3-\frac{1}{x}=0. \end{align}

Solve the second equation for $$x^4$$ to get $$x^4=\frac{1}{4c}$$. Substitute this into the first equation to get $$\ln x = \frac{1}{4}$$, which implies $$x=e^{1/4}$$. Finally, substitute this equation for $$x$$ into $$\frac{1}{4}=\ln x=cx^4$$ to find that $$c=\frac{1}{4e}$$.