# Maximum cubic function

Let $f(x)=x^{3}+ax^{2}+bx+c$ with a, b, c real.

Show that

$$\frac{1}4 \le \max_{-1 \le x \le 1\hspace{2mm}} |f(x)|=M$$

and find all cases where equality occurs.

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I take it $M$ is just a symbol for the maximum absolute value of $f$, and one is supposed to prove that this maximum is at least $1/4$, regardless of the choice of real $a$, $b$, and $c$. – Gerry Myerson Feb 7 at 22:22
@GerryMyerson: yes, I figured that out :-) – robjohn Feb 7 at 22:23
Equality occurs for $\pm(x^3-(3/4)x)$. – Gerry Myerson Feb 7 at 22:26
@GerryMyerson: The hard part is showing that is the smallest. – robjohn Feb 7 at 22:30
Interesting that prior to my voting this question had 2 votes and 4 favorites. – JSchlather Feb 7 at 23:21
Note that $$\max_{[-1,1]}\,\left|\,x^3-\tfrac34x\,\right|=\tfrac14\tag{1}$$ Considering the symmetry about $0$ of the domain, we have for any $t\in[0,1]$ $$\max_{\{t,-t\}}\,\left|\,x^3+ax^2+bx+c\,\right|=\left|\,t^3+bt\,\right|+\left|\,at^2+c\,\right|\tag{2}$$ Using $(2)$, it is obvious that $$M_b=\max_{[-1,1]}\,\left|\,x^3+bx\,\right|\le\max_{[-1,1]}\,\left|\,x^3+ax^2+bx+c\,\right|\tag{3}$$ It is straightforward to compute $$M_b=\left\{\begin{array}{} 2(-b/3)^{3/2}&\text{if }b\in\left[-3,-\tfrac34\right]\\ |\,1+b\,|&\text{otherwise} \end{array}\right.\tag{4}$$ and $M_b$ reaches a minimum of $\frac14$ only at $b=-\frac34$. For any other value of $b$, $(3)$ says that $$\max_{[-1,1]}\,\left|\,x^3+ax^2+bx+c\,\right|\ge M_b>\tfrac14\tag{5}$$ Setting $b=-\frac34$ and $t=\frac12$ in $(2)$ yields $$\max_{[-1,1]}\,\left|\,x^3+ax^2-\tfrac34x+c\,\right|\ge\tfrac14+\left|\,\tfrac14a+c\,\right|\tag{6}$$ and this can be $\frac14$ only if $c=-\frac a4$.
At $|x|=\frac12$, $$\left|\,x^3+ax^2-\tfrac34x-\tfrac a4\,\right|=\tfrac14\tag{7}$$ However, at $x=\pm\frac12$, the derivative of $x^3+ax^2-\frac34x-\frac a4$ is $\pm a$. Therefore, the maximum of $\left|\,x^3+ax^2-\tfrac34x-\tfrac a4\,\right|$ will be greater than $\frac14$ unless $a=0$.
Thus, $$\max_{[-1,1]}\,\left|\,x^3+ax^2+bx+c\,\right|\ge\tfrac14\tag{8}$$ where equality holds only for $x^3-\frac34x$.
 is the second equation to be an inequality? – Maesumi Feb 7 at 23:38 @Maesumi: no, it is an equation. If $x^3+bx$ and $ax^2+c$ cancel (have different signs) at $x=t$, then they reinforce (have the same sign) at $x=-t$, and vice-versa. Note that it is a $\max$ over two points, $\{t,-t\}$. – robjohn♦ Feb 8 at 13:19