Use this tag for questions about differential and integral calculus with more than one independent variable. Some related tags are (differential-geometry), (real-analysis), and (differential-equations).

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10 views

A book like Michael Spivaks Calculus, for multivariate Calculus.

Is there a book like Michael Spivaks Calculus, that is for Multivariate Calculus? That is a "real analysis" multivariate calculus book?
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2answers
21 views

Evaluate 2D integral (by change of variable)

The question asks to evaluate integral $$\iint_D \Big[3-\frac12( \frac{x^2}{a^2}+\frac{y^2}{b^2})\Big] \, dx \, dy \ $$ where D is the region $$\frac{x^2}{a^2}+\frac{y^2}{b^2} \le 4 $$ I believe ...
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0answers
7 views

Prove that exists $\epsilon >0$ such that $S\cap C\cap B((0,0,0),\epsilon)=\{(0,0,0)\}$

I can't find the way to do this exercise. We consider $S=\{(x,y,z) \in \mathbb{R^3}: f(x,y,z)=0 \}$, where $f$ is a $C^1$function on $\mathbb{R^3}$ such that $f(0,0,0)=0$ and $\nabla ...
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0answers
8 views

Parametrisation of surface

Let $K= \{ (x,y,z) \in \mathbb{R}^3 : \sqrt{x^2+y^2} \leq z,\,\, x^2 + y^2 + z^2 = 1 \}$. I need a parametrisation of $K$ in order to calculate the flux of some function through $K$. I'm not sure ...
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0answers
11 views

Is this the proper way to differentiate a vector with scalar?

$\vec{r}(t)=(r_0+kt)\cdot\begin{pmatrix}\sin(\omega t)\\\cos(\omega t)\end{pmatrix}$ $\vec{r}(t)=\begin{pmatrix}r_0\sin(\omega t)+kt\space \sin(\omega t)\\r_0\cos(\omega t)+kt\space \cos(\omega ...
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0answers
16 views

Calculate the flux coming out of a surface

Let F(x,y,z)=(2xy(z-2),x^2(z-2),x^2y) be a vector field and $\Sigma $ the surface defined as the portion of cone x^2+y^2=(z-2)^2 ...
2
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0answers
28 views

Determine the volume of $A:=\{(x,y,z)\in \mathbb R^3 : \sqrt{x^2+y^2}\leq f(z)\}$

Let $f\in L^2(\mathbb R)$ and $f\geq0$. Determine $A:=\{(x,y,z)\in \mathbb R^3 : \sqrt{x^2+y^2}\leq f(z)\}$. "Normal" substitution $(x=rcos(\phi),y=rsin(\phi))$ did not help a lot, since I dont have ...
2
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2answers
58 views

Area enclosed by cardioid using Green's theorem

Let $$\gamma(t) = \begin{pmatrix} (1+\cos t)\cos t \\ (1+ \cos t) \sin t \end{pmatrix}, \qquad t \in [0,2\pi].$$ Find the area enclosed by $\gamma$ using Green's theorem. So the area enclosed by ...
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1answer
26 views

Triple integral over a region [on hold]

My problem is to evaluate the following: $$\iiint_D{6xy\space dV},$$ where D is the solid, bounded from above by the plane $z=1+x+y$, below the region in the $xy$-plane, and by the curves ...
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0answers
13 views

Absolute square in deriving Fourier transform variance

I'm having some trouble understanding how to derive the variance of the Fourier transform. This is for an image, i.e., it's a 2D transform. The variance is $|\hat{I}(\xi,\eta)|^2$, the absolute ...
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1answer
17 views

If $∇f(a)\cdot y ≤ 0$ for every vector $y$, why does $\nabla f(a)$ have to be zero?

If $f$ is differentiable at every point in $B(a)$ and $f(x)≤f(a)$ for all $x$ in $B(a)$, prove that $∇f(a)=0$. I actually did some work and found out that $∇f(a)\cdot y ≤ 0$ for every vector $y$. ...
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1answer
21 views

How to determine a function whose minima falls on a specified curve?

I have a family of curves given by $g(x,y)=C_0 yx^{-n}$. How can I determine the function $f(x,y)$ for the family of curves that satisfies the condition that the local minima $\frac{\partial ...
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1answer
26 views

changing the order of double integration

I am a little bit stucked about the changing the order of a double integration. $\displaystyle \int_{0}^{\infty}\int_{t-n}^{t}f(n,s)dsdn$ I try to represent the upper and lower bounds by a graphic ...
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0answers
17 views

Let $f$ be a scalar field such that $f ' (a ;-y)$ exsits [on hold]

Let $f$ be a scalar field where derivative of $f$ at point a with respect to vector $-y$ exists, $f '(a;-y)$ exists. Is it always true for any nonzero vector $y , f '(a;-y) = - f '(a;y)$?
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1answer
10 views

Image of a circumference by a vector function

I'm doing this exercise and I don't know how to finish. Consider the vector function $F(x,y) = (x^2+y^2, 2xy)$. Determine the image of the circumference $x^2+y^2 = a^2$, $a>0$, and obtain the ...
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1answer
24 views

Assume $f$ is differentiable at every point of $B(a)$ and $f(x)$ is less than or equal to $f(a)$

Over the scalar field, If $f$ is differentiable at every point in $B(a)$ and $f(x)$ is less than or equal to $f(a)$, prove why gradient of $f(a)$ is $0$. Just don't understand how to start with,
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1answer
19 views

Derivative over scalar field with respect to fixed point proof.

Prove there is no such scalar field that $f '(a;y) >0$ for fixed point $a$ and every non-zero vector $y$. I posted this question but some of you pointed out that it is not clear. So, $f ' (a;y)$ ...
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1answer
17 views

Let $f: \mathbb R \times \mathbb R^n \rightarrow \mathbb R^n$ given as $f(t,y(t))$. How I understand:“$f$ has continuous partial $y$-derivatives”?

Suppose I have a function $f: \mathbb R \times \mathbb R^n \rightarrow \mathbb R^n$ given as $f(t,y(t))$. Then how should it be understood "$f$ has continuous partial $y$-derivatives" ? Should it be ...
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2answers
16 views

plotting of level curves using pen and paper

I want to plot the level curve for the function $f(x,y)=\frac{y^2-x^4}{y^2+x^4}$ . I tried by substituting $f(x,y)=k$. But I am Unable to draw it using paper and pen. Kindly help me.
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1answer
21 views

multivariable limit problem

I have a confusion regarding this problem. Problem: $\displaystyle f(x,y)=\frac{\sin^2|x+2y|}{x^2+y^2}$ is continuous for all $(x,y)\neq (0,0)$. True or false? I think that the limit does not exist ...
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0answers
10 views

Double integral discontinuity question

In lecture we were doing the problem $\int {1 \over \sqrt{xy}}\,dxdy$ over the region $[0,1] \times [0,1]$. Since the function is undefined when $x=0$ or $y=0$ we took the limit as $\delta,\epsilon ...
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0answers
7 views

mean value theorem for scalar field

I just want to make sure that Mean value theorem for scalar field works same as one- dimensional mean value theorem. Usually, my book explains Mean value theorem for scalar field on interval [0.1]. ...
2
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0answers
6 views

Mean value theorem and scalar field proof

Assume that f′(x;y)=0 for every x in some n-ball B(a) and for every vector y. Use the mean value theorem to prove that f is constant on B(a). And if f′(x;y)=0 for a fixed vector y and for every x in ...
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1answer
27 views

Finding the unit normal vector

Q. Consider the following vector function. $$ r(t)= \langle 6\sqrt{2}t,e^{6t},e^{-6t} \rangle $$ Find the unit tangent and unit normal vectors T(t) and N(t). I found $$T(t)= ...
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0answers
12 views

Can anyone help me prove derivative of scalar field using mean value theorem?

Assume that f′(x;y)=0 for every x in some n-ball B(a) and for every vector y. Use the mean value theorem to prove that f is constant on B(a). And if f′(x;y)=0 for a fixed vector y and for every x ...
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0answers
22 views

There is no scalar field such that $f '(a)>0$ for fixed $a$ and for every nonnegative vector $y$ [on hold]

I am trying to prove this. But can't think of how I should start. Anyone has some ideas? and why is there a scalar field $f'(a)>0$ for every $a$ and for fixed vector $y$ ? can anyone give me an ...
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1answer
37 views

Find limit of $f(x,y) = \frac{x^3\cdot y-x \cdot y^3-x}{1-x \cdot y}$ where $(x,y)$ approaches $(0,0)$ [on hold]

$$ \lim\limits_{(x, y)\to (0, 0)}\frac{x^3y-xy^3-x}{1-xy} $$ I don't know how to solve this limit. All help is appreciated.
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0answers
20 views

Volume of $y = 6\sqrt{\sin(x)}$ rotated around $y$-axis using triple integrals

The problem is to find the volume of $y = 6\cdot \sqrt{\sin (x)}$ rotated around the $y$-axis when $0 \leq y \leq 6$. I know this can be done by the sv-calc method of volumes of revolution but I ...
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1answer
14 views

How can I find line segment connecting two vectors?

Let $S$ be a subset of $\mathbb{R}^n$. it is called convex if for all pairs of $a$, $b$, line segment from $b$ to $a$ is element of $S$. And it is given that $at+(1-t)$ is line segment between two ...
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3answers
33 views

Multivariable: Show the limit is $0$.

I already proved that for $\alpha =1$ the limit doesn't exists. Now I need to show that for $\alpha > 1$ the limit does exists and equals $0$. $$\lim_{(x,y)\to (0,0)} \frac{\left|x\right|^\alpha ...
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0answers
48 views

Find the volume bounded by the sphere $x^2+y^2+z^2=4$ and $x^2+y^2-2x=0$

This question appeared on my calculus exam yesterday. I don't know how to do it: Find the volume bounded by the sphere $x^2+y^2+z^2=4$ and $x^2+y^2-2x=0$. My attempt: First, I realised that the ...
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1answer
29 views

Solution of a Partial Differential Equation

Problem statement Solve $\frac{\partial f}{\partial x}-x\frac{\partial f}{\partial y}=y$ using the change of variables $\left\{\begin{matrix} u=ax^2+y \\ v=x \end{matrix}\right.$ for a suitable ...
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1answer
56 views

Find $F'(t)$, where F is an integral

I need to find $F'(t)$, where $F(t)=\int_{[0,t]^2}e^{\frac{tx}{y^2}}dxdy$. My first approach: Let's observe that $\int e^{\frac{tx}{y^2}}dx=\frac{y^2}{t}e^{\frac{tx}{y^2}}+C$. So I get: ...
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1answer
11 views

Cylindrical limits of integration for a particular triple integral

In cylindrical coordinates, what would be the limits of integration for the triple integral serving to find the volume of the region in $\mathbb R^3$ bounded by: $x^2 + y^2 = y$ and the sphere of ...
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2answers
63 views

Find all planes which are tangent to a surface

I'm given the surface $z=1-x^2-y^2$ and must find all planes tangent to the surface and contain the line passing through the points $(1, 0, 2)$ and $(0, 2, 2).$ I know how to calculate tangent planes ...
2
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2answers
29 views

Can every differentiable scalar function be written as a divergence of some vector field?

My question is simple: can every differentiable function $f$ defined on a bounded, connected subset of $\mathbb{R}^3$ be written as a divergence of some vector field ? That is, given the vector field ...
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1answer
28 views

Rectangles in one dimension

I have to prove the following proposition : Show that the intesection of two rectangles in $\mathbb{R}^{n}$ is either the vaccum or is another rectangle. My attempt: I one is embeded in the other ...
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0answers
8 views

Bound all $k$-th derivatives by directional derivatives of order $k$

Assume $f\in C^k(\mathbb{R}^n)$, $x\in\mathbb{R}^n$, and $|(\partial_\xi)^kf(x)|\leq 1$ for all $\|\xi\|=1$. Which bounds do we have for $|\partial^\alpha f(x)|$ when $|\alpha|=k$? For example, if ...
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1answer
34 views

Interpreting limit notations

My question is: Are the following notations equivalent or not: $$(1)\;\;\;\;\;\;\text{When}\;||\textbf{x}||\rightarrow 0,\;\text{then}\;\;\;\frac{f(\textbf{x})}{||\textbf{x}||}\rightarrow0$$ ...
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1answer
18 views

$F(x,y)=2x^4-3x^2y+y^2$. Show that $(0,0)$ is local minimum of the Reduction of F for every linear line that passes through $(0,0)$.

first of all I checked if (0,0) is critical point $Df(0,0)=(8x^3-6xy,-3x^3+2y)| = (0,0) $ now my idea was to replace $y$ with $xk$ because of the reduction of $F$ ,and find the hessian matrix to ...
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1answer
12 views

Proving that $2$-D parabolic coordinates are orthogonal

How can we prove that the parabolic coordinate system in two dimensions is orthogonal? I tried using the dot product, but don't know where to start or what basis vectors can be used in two dimensions. ...
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0answers
15 views

Setting up a double integral in terms of x and y to find flux

I am presented with the following problem, and it wants me to set up the double integral in terms of x and y, but I have no idea on how to continue solving this one, any ideas? Set up a double ...
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15 views

Can this multivariable function exist?

(3) Is there a function of two variables whose z = 0 level curve consists exactly of the circles $x^2$ + $y^2$ = 4 and $x^2$ + $y^2$ = 10? If so, what is an example? If not, why not? I initially ...
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1answer
20 views

Independepnce of path in a closed curve line integral

Let $f(t)$ be a continuous function. Let $C$ be a smooth closed curve. Show that $$\oint\limits_C xf(x^2 + y^2)\,dx + y f(x^2 + y^2)\,dy = 0$$ Hint: Remember that $f(t)$ has a primitive function ...
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1answer
31 views

Proof of transformation law for double integrals

The second volume of Apostol's Calculus seems rather circumspect in its discussion of the change of variables formula for double integrals. Section 11.29 offers a proof under the following very ...
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4answers
201 views

Chain rule for partial derivatives intuition

Can somebody give me an intuitive explanation for the above equations. I'm not sure how they come about and how they can be perceived logically. $$\frac{\partial z}{\partial s} =\frac{\partial ...
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0answers
5 views

Partial derivative notation inside an integral - what would be normal?

What would be the most idiomatic way of writing the following idea? $$x^{t+u} = \int_0^x\int_0^{x}\frac{\partial (y^t)}{\partial y}\cdot \frac{\partial (z^u)}{\partial z}dz dy$$ for $\Re(x)>0$. ...
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0answers
14 views

Gaussian variance estimation via spectral decomposition

I was given a dataset (a mat file) of 100,000 observations, each with 50 dimensions (coordinates). Denote matrix $X$ is a 100,000x50 matrix in which each column was generated according to: ...
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0answers
13 views

Kneser Inequality in multivariables

Based on the Kneser Inequality ("Polynomials and Polynomial Inequalities", p. 260) one has $\Vert q \Vert_{[-1, 1]} \Vert r \Vert_{[-1, 1]} \leq C(n, m) \Vert q r \Vert_{[-1, 1]}$ where all norms ...
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0answers
27 views
+50

Parametric equations for hypocycloid and epicycloid

Suppose that the small circle rolls inside the larger circle and that the point $P$ we follow lies on the circumference of the small circle. If the initial configuration is such that $P$ is at ...