Question Number 2548 by Filup last updated on 22/Nov/15
$$\mathrm{For}\:\mathrm{a}\:\mathrm{function}\:{y}={f}\left({x}\right), \\ $$$$\mathrm{inflection}\:\mathrm{points}/\mathrm{stationary}\:\mathrm{points}\:\mathrm{are} \\ $$$$\mathrm{when}\:\:\frac{{df}}{{dx}}=\mathrm{0}. \\ $$$$ \\ $$$$\mathrm{For}\:\mathrm{a}\:\mathrm{function}\:{z}={f}\left({x},\:{y}\right),\:\mathrm{can}\:\mathrm{you}\:\mathrm{find} \\ $$$$\mathrm{these}\:\mathrm{points}\:\mathrm{through}\:\mathrm{a}\:\mathrm{similar}\:\mathrm{method}? \\ $$$$ \\ $$$$\mathrm{Is}\:\mathrm{it}\:\mathrm{something}\:\mathrm{like}\:\frac{\partial{f}}{\partial{x}}=\mathrm{0}\:\mathrm{and}\:\frac{\partial{f}}{\partial{y}}=\mathrm{0}? \\ $$
Answered by Yozzi last updated on 22/Nov/15
$${Let}\:{f}\:{be}\:{a}\:{function}\:{of}\:{two}\:{variables} \\ $$$${whose}\:{first}\:{and}\:{second}\:{partial}\: \\ $$$${derivatives}\:{are}\:{continous}\:{on}\:{some}\:{open} \\ $$$${disc}\:{B}.\:{Suppose}\:{further}\:{that}\:{at}\:{the}\:{point} \\ $$$$\left({a},{b}\right) \\ $$$${f}_{{x}} =\frac{\partial{f}}{\partial{x}}=\mathrm{0}\:\:\:{and}\:\:\:{f}_{{y}} =\frac{\partial{f}}{\partial{y}}=\mathrm{0}.\: \\ $$$${Let}\:{p}=\frac{\partial^{\mathrm{2}} {f}}{\partial{x}^{\mathrm{2}} }={f}_{{xx}} \:,\:{q}=\frac{\partial^{\mathrm{2}} {f}}{\partial{y}^{\mathrm{2}} }={f}_{{yy}} \:{and}\:{r}=\frac{\partial^{\mathrm{2}} {f}}{\partial{y}\partial{x}}={f}_{{xy}} \\ $$$$\left({a},{b}\right)\:{is}\:{a}\:{local}\:{minimum}\:{if}\:{pq}−{r}^{\mathrm{2}} >\mathrm{0}\:\:{and}\:{p}>\mathrm{0}\:\left({or}\:{q}>\mathrm{0}\right), \\ $$$$\left({a},{b}\right)\:{is}\:{a}\:{local}\:{maximum}\:{if}\:{pq}−{r}^{\mathrm{2}} >\mathrm{0}\:\:\:{and}\:{p}<\mathrm{0}\:\:\left({or}\:\:{q}<\mathrm{0}\right) \\ $$$$\left({a},{b}\right)\:{is}\:{a}\:{saddle}\:{point}\:{if}\:{pq}−{r}^{\mathrm{2}} <\mathrm{0} \\ $$$${pq}−{r}^{\mathrm{2}} =\mathrm{0}\:{is}\:{inconclusive}. \\ $$
Commented by Filup last updated on 22/Nov/15
$$\mathrm{My}\:\mathrm{question}\:\mathrm{is} \\ $$$$\mathrm{what}\:\mathrm{is}\:\mathrm{an}\:\mathrm{open}\:\mathrm{disc}\:\mathrm{and}\:\mathrm{what}\:\mathrm{is}\:{f}_{{xy}} ? \\ $$$$\mathrm{is}\:\:\frac{\partial^{\mathrm{2}} {f}}{\partial{x}\partial{y}}\:\:\mathrm{differentiation}\:\mathrm{with}\:\mathrm{two}\:\mathrm{variables}? \\ $$
Commented by Yozzi last updated on 22/Nov/15
$${f}_{{x}} \:{is}\:{partial}\:{differentiation}\:{of}\:{f}\:{w}.{r}.{t}\:\:{x} \\ $$$${f}_{{xy}} =\frac{\partial}{\partial{y}}\left({f}_{{x}} \right)=\frac{\partial}{\partial{y}}\left(\frac{\partial{f}}{\partial{x}}\right)=\frac{\partial^{\mathrm{2}} {f}}{\partial{y}\partial{x}}\:\left({mixed}\:\mathrm{2}{nd}\:{partial}\:{derivative}\right) \\ $$$${An}\:{n}−{dimensional}\:{open}\:{disc}\:{of} \\ $$$${radius}\:{r}\:{is}\:{the}\:{collection}\:{of}\:{points}\:{less} \\ $$$${than}\:{a}\:{distance}\:{of}\:{r}\:{away}\:{from}\:{a}\:{fixed} \\ $$$${point}\:{in}\:{Euclidean}\:{n}−\:{space}.\:\left({Wolfram}\:{Alpha}\right) \\ $$$${If}\:{n}=\mathrm{1}\:{for}\:{example},\:{this}\:{is}\:{an}\:{open}\: \\ $$$${interval}\:\left({s},{t}\right).\:{So},\:{for}\:{n}=\mathrm{2}\:{this}\: \\ $$$${defines}\:{a}\:{collection}\:{of}\:{points}\:{in}\:{a}\:{circle}\:{containing} \\ $$$${the}\:{point}\:\left({a},{b}\right)\:{where}\:{both}\:{f}_{{x}} \:{and}\:{f}_{{y}\:} \\ $$$${are}\:{zero}.\:{In}\:{this}\:{case},\:{once}\:{the}\:\mathrm{1}{st} \\ $$$${and}\:\mathrm{2}{nd}\:{partial}\:{derivatives}\:{of}\:{f}\:{exist} \\ $$$${and}\:{the}\:{both}\:{f}_{{xy}} \:{and}\:{f}_{{yx}\:\:} \:{are} \\ $$$${continuous}\:{for}\:{the}\:{set}\:{of}\:{points}\:\left({x},{y}\right) \\ $$$${in}\:{the}\:{disc},\:{these}\:{two}\:{mixed}\:{partial} \\ $$$$\mathrm{2}{nd}\:{derivatives}\:{are}\:{equal},\:{i}.{e}\:{f}_{{xy}} ={f}_{{yx}} . \\ $$
Commented by Filup last updated on 22/Nov/15
$$\mathrm{I}\:\mathrm{see}.\:\mathrm{Very}\:\mathrm{interesting}.\:\mathrm{I}\:\mathrm{must}\:\mathrm{go}\:\mathrm{learn} \\ $$$$\mathrm{multi}−\mathrm{variable}\:\mathrm{calculus}! \\ $$