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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0
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1answer
31 views

Using partial derivates to solve for constant?

Suppose that $f$ is a function of $x$ and $y$ such that (partial derivative in respect to $x$) $f_x = x+2y$ and (partial derivative in respect to $y$) $f_y = a + 3y$ where a is a constant. What does ...
1
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0answers
17 views

Comparing double integral and single integral (finding which one is larger)

Hi I have two equations $$F(a):=\int^{\bar{x}}_{a}\int^{x}_{\underline{x}}2(x-c)\, dy \, dx$$ $$G(b):=\int^{\bar{x}}_{b}(y-c)\,dy$$ where $a,b,c \in [\underline{x}, \bar{x}]$ and $a>b>c$ I ...
0
votes
2answers
24 views

Equation for Tangent Plane and Linear Approximation

I need help finding an equation for a tangent plane to the following graph at the point $(1,2,5)$: $$z=f(x,y)=x^2+2xy$$ For this question, I got $z=6x+2y-5$ as the tangent plane. Can someone verify ...
2
votes
1answer
37 views

literature on advanced calculus [on hold]

I need your opinions on this particular textbook: Advanced Calculus by Robert C. Buck. In my first year in college I finished two semesters of single-variable calculus and now I'm looking for a proper ...
7
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0answers
43 views

What is the Exterior Derivative Trying to Do?

$\newcommand{\R}{\mathbf R}$ Consider a smooth function $f:\R^n\to\R$ and let $Df:\R^n\to \R^{n*}$ be the map which takes a point $\mathbf a\in R^n$ to the linear map $Df_{\mathbf a}:\R^n\to \R$. ...
1
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1answer
40 views

How find $I= \int_{x=0}^{ \frac{1}{2} } \int_{y=x}^{1-x} ( \frac{x-y}{x+y})^{2}\, dy\,dx$

In $$I= \int_{x=0}^{ \frac{1}{2} } \int_{y=x}^{1-x} \left( \frac{x-y}{x+y}\right)^{2} \,dy\,dx$$ follow the change of variables on $x= \frac{1}{2} (r-s),y= \frac{1}{2} (r+s)$ and find$I$ My try ...
0
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0answers
14 views

Finding the angle between two vectors which involves gradients

Let $f(x,y), g(x,y)$ be two functions with the following property: For every constant $x$, the function in the variable $y$ is decreasing. In addition: $$ f(x,x^2 ) =3 , \, g(2y-3,y)=4 $$ Prove ...
-1
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0answers
23 views

An equation to describe a bowl shape in 3D [on hold]

I was wondering how would the 3D graph of bowl shape with positive x , y , z axis be plotted and what will be its equation. I was wondering around this and I am really excited to get this equation ...
1
vote
1answer
22 views

conservative vector fields - need a counterexample in $\mathbb{R}^2$?

I am given with the following statement: If $\vec{F}(x,y)$ is conservative in a region $A$ and in a region $B$ , then it is also conservative in $A\cup B$ . I know the statement is incorrect, but ...
1
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0answers
50 views

Is the Schwarz inequality a special case of the Cauchy-Schwarz inequality?

Given two vectors $\mathbf{x},\mathbf{y}$ in $\mathbb{R}^n$, we all know that:$$\left | \mathbf{x}\cdot\mathbf{y} \right | \le \left \| \mathbf{x} \right \| \cdot\left \| \mathbf{y} \right \|$$ ...
2
votes
2answers
2k views

Finding a vector parametric equation given P and Q equations?

Find a vector parametric equation $r⃗(t)$ for the line through the points $P=(3,5,4)$ and $Q=(1,4,7)$ for each of the given conditions on the parameter $t$. If $r⃗ (0)=(3,5,4)$ and $r⃗ (7)=(1,4,7)$, ...
1
vote
0answers
14 views

Trigonometric parametrization of a genus g surface?

It is possible to find functions $\phi, \psi \in \mathbb{R}[sin(x), sin(y), cos(x), cos(y)]$, so that $S^2 = \phi( [0,1]^2)$ and $\psi( [0,1]^2)$ is a torus. Is it possible find, for any genus g, ...
1
vote
4answers
41 views

How to use Cross Product Properites to do proof

How do I proceed with a proof for this question? Prove that: \begin{equation} (a \times b) \cdot (c \times d) = \begin{vmatrix} a \cdot c & b \cdot c \\ a \cdot d & b \cdot ...
0
votes
1answer
36 views

Local extrema in special directions

I am looking for the extrema of a function $G(y_1,y_2,y_3,y_4)$ subject to the constraint $y_1 = y_4 + y_2y_3.$ We know that $G$ is defined if $(y_2,y_3,y_4)$ is in the cylinder $\mathbb{D} \times ...
0
votes
3answers
23 views

Problem about planes

Say we have $2x+3y+3z=0$ which is a plane. Does that plane have infinite dimensions (it is a 2D "object" — forgive me as I am not a mathematician — but each side has infinite length) or is it just the ...
0
votes
1answer
25 views

Statement about the gradient

Let $f \in \mathcal C^1(\mathbb R^n).$ If there exists $u \in S^{n-1}$ such that $$\nabla f(x) \cdot u \geq 0 \quad\forall x\in \mathbb R^n,$$ then $f(u) \geq f(0)$. How to prove this statement?
2
votes
1answer
28 views

Example where partial derivatives commute but are not continuous.

I am looking for an example of a function $f:\mathbb R^2\to\mathbb R$ such that there is a point $x\in\mathbb R^2$ with the following properties: 1) All partial derivatives of second order exist in a ...
2
votes
2answers
22 views

Necessary condition for local maximum

Let $\Omega\subset \mathbb{R}^n$ open, bounded and let $f:\Omega\to\mathbb{R}$ be a $C^2$-function. I want to prove: Necessary for a interior maximum $x_0\in\Omega$ is that $D^2f(x_0)$ is negative ...
5
votes
1answer
336 views

Find the volume of the region bounded by the planes $ z=8-y^2, y = 8-x^2, x=0, y=0, z=0$

I figured out the bounds for z: $z=0$ to $z=8-y^2$ The bounds for y: $y=0$ to $y=8-x^2$ The bounds for x: $x=0$ to $x=\sqrt{8}$ (Since $8-x^2 = 0$) So, the volume by using triple integral: ...
1
vote
1answer
22 views

Applying chain rule on a quadratic form

I'm trying to apply the chain rule on a quadratic form: $\frac{dx^TAx}{dx}=\frac{dx^T(Ax)}{dx}=\frac{dx^T(Ax)}{dAx}\frac{dAx}{dx}=\frac{dx^T(Ax)}{dAx} A$ But I'm stuck here. I think ...
0
votes
3answers
101 views

Books various maths subjects [on hold]

I am a Civil Engineering student and i am planning on following physics in my career.I want to be ready for the advanced undergraduate courses that i will attend to,so i need to learn Differential ...
0
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0answers
19 views

Evaluating surface area

These are two exercises from Apostol calculus that I am struggling to set up the integrals correctly. The biggest problem for me is finding the correct region $T$ under the surface $S$. Compute the ...
1
vote
2answers
24 views

What is the area of the part of the surface $z=yx$ bounded by $x^2+y^2=1$?

A parametrization of the part of the surface $z=yx$ bounded by $x^2+y^2=1$ is \begin{align} x &= u \cos v \\ y &= u \sin v \\ z &= \frac12 u^2 \sin 2v, \end{align} or $$r(u,v)=u \cos v \, ...
1
vote
1answer
13 views

Line integral of vector field/Why doesn't my solution work?

The question in its entirety: Determine for which constants A & B the vector field $$\mathbb{F} = (Axln(z))\mathbb{i} + (By^2z)\mathbb{j} + ((\frac{x^2}{z})+y^3)\mathbb{j}$$ is conservative. If ...
1
vote
1answer
16 views

Need help finding tangent plane to a surface

I'm doing a Calc III homework problem, and I cannot seem to figure out what the correct solution is. $$ \text{Find the equation of the tangent plane to the surface }z = 9 y^{2} - 9 x^{2}\text{ at the ...
6
votes
1answer
209 views

How to solve this differential equation for $y$ in terms of $x$ and $k$

$$yy'+\frac yx+k=0$$ How to solve this differential equation for $y$ in terms of $x$ and $k$ where $k$ is a parameter of $x$? $y(x)=y$ is a function and $x(k)=x$ is a gamma function
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0answers
19 views
1
vote
1answer
387 views

surface area over a tetrahedron

Compute $\int\int xy dS$, where $S$ is the surface of the tetrahedron with sides $z=0,y=0,x=0$ and $z=1-x-y$. I evaluated $\sqrt{3}\int_0^1\int_0^{1-x}xy dydx$ and got the result as ...
0
votes
1answer
31 views

Show that $\|f\|^{2}$ attains a minimum value on the interior of $B$

I am looking for any help, hints, or suggestions in how to go about this problem from a previous qualifying exam. We are given a smooth mapping $f: U \rightarrow \mathbb{R}^{n}$ whose differential ...
2
votes
1answer
38 views

What is the volume inside $S$, which is the surface given by the level set $\{ (x,y,z): x^2 + xy + y^2 + z^2 =1 \}$?

The solution given uses a linear algebraic argument that doesn't seem very instructive -- and may not even be correct, I think. We notice from the equation, that the surface is a quadratic form, ...
1
vote
2answers
105 views

Limits in multivariable functions

I tried to evaluate this limit but I can't see any limited function here (the limit exists). $\lim\limits_{(x,y)\to(0,0)}\frac{2x^2y}{x^4 + y^2}$ Thank you.
2
votes
1answer
19 views

Choosing a vector normal to a jordan curve that points “inside”

Let $\gamma=\partial K_1(0,0)$ be the circle with radius $r=1$ and origin $(0,0)$ in $\mathbb R^2$. Then for any $t_0$ we have $\gamma'(t_0)\neq \begin{pmatrix} 0 \\ 0\end{pmatrix}$. Let ...
0
votes
0answers
22 views

Gradient and invariance under change of basis

I intuitively would be inclined to believe that the gradients $\nabla F_i$ of the components $F_1,\ldots,F_3$ of a vector field $\mathbf{F}:A\subset\mathbb{R}^3\to\mathbb{R}^3$, $\mathbf{F}\in ...
1
vote
2answers
26 views

A Point on Curve Where Tangent is Parallel

How do I find a point on a curve, $r=\langle 2,3t,5t^3\rangle$ in which tangent line is parallel to some plane. I thought to find the derivative first, but having trouble there as well. Anyway, this ...
0
votes
0answers
43 views

Is the unit square a $2$-manifold in $\mathbb{R}^2$?

I'm using the following definition of a (smooth) manifold: It's from J.Munkres "Analysis on Manifolds". This is an exercise taken from this book: Is the unit square $[0,1]\times [0,1]$ a ...
1
vote
1answer
41 views

Curl: invariant under change of basis or not?

I wondered how the curl$$\text{rot}\mathbf{F}=\left( \begin{array}{ccc}\partial_y F_3-\partial_z F_2 \\ \partial_z F_1-\partial_x F_3 \\ \partial_x F_2-\partial_y F_1 \end{array} \right)$$of a vector ...
0
votes
0answers
17 views

Finding the curvature from a set of datapoints

I have a set of 1. 1-d 2. 2-d data. I want to find the curvature at each single point. Till now I was using difference technique to find out the curvature, i.e, central difference at middle and ...
4
votes
1answer
68 views

Condition for Continuity (two variable)

I came across the following question while studying for quals. This one is from a previous qualifier. I have a few ideas (which I'll mention below), but am stuck on how to complete the problem. Any ...
2
votes
1answer
32 views

derivative of a recursive vector-valued function

I have a recursive vector-valued function $$\mathbf{y}(t)=\mathbf{W}\mathbf{y}(t-1).$$ To compute the derivative of $\mathbf{y}(t)$ with respect to $\mathbf{W}$, do I need to use the product rule? ...
4
votes
2answers
44 views

If $\langle f'(x) \cdot v , v \rangle > 0$ then $f$ is injective

Question: Let $f: U \to \mathbb R^m$ differentiable at the convex set $U \subseteq \mathbb R^m$. If $$\langle f'(x) \cdot v , v \rangle > 0 , \,\,\, \forall\,\, x \in U, v \neq 0 \in \mathbb ...
0
votes
0answers
17 views

Parameterization of an ellipsoid in spherical coordinates

\begin{align*} 25x^2+16y^2+z^2=1 \\ \frac{x^2}{4^2} + \frac{y^2}{5^2} + \frac{z^2}{20^2} = \frac{1}{20^2} \end{align*} The spherical coordinates are defined as, \begin{align*} x &= \rho ...
0
votes
0answers
31 views

Induction on derivatives

I have troubles understanding this induction proof: Let $$g(x) = \vert x \vert^{2k+1}$$ Show by induction: $$\frac{\partial ^N g(x)}{\partial x_{i_1} \dots \partial x_{i_N})} = cx_1n \dots x_iN \vert ...
0
votes
1answer
39 views

Application of inverse function theorem?

I am not completely sure if this a direct consequence of the inverse function theorem. Assume that we have a function $f: \mathbb{R}^2 \rightarrow \mathbb{R}$ that we can write in terms of ...
0
votes
1answer
38 views

Sum of power functions over a simplex

Let $d \ge 1$ be a positive integer. Let $n,m$ be another positive integers subject to $m\ge n+d$. Let $\vec{x} := (x_1,x_2,\cdots,x_d)$ be real numbers such that all of them cannot be equal to ...
1
vote
1answer
40 views

Verify $\frac {\partial B} {\partial T} =$ $\frac{c}{(e^\frac{hf}{kT}-1)^2}\frac{hf}{kT^2}e^\frac{hf}{kT}$

Find an expression for $\frac {\partial B} {\partial T}$ applied to the Black-Body radiation law by Planck: $$B(f,T)=\frac{2hf^3}{c^2\left(e^\frac{hf}{kT}-1\right)}$$ The correct answer (I believe) ...
1
vote
2answers
29 views

second derivative of the composition of two multivariable functions

Let $U \subset \mathbb{R}^n$ be open, and let $\gamma: \mathbb{R} \to U$ and $f: U \to \mathbb{R}$ be to functions that are differentiable at least twice. I want to show that $\frac{d^2}{dt^2}(f ...
1
vote
1answer
35 views

Coordinate calculation on a unit sphere

I'm writing a first person 3D game and I do not know the math behind what I need. I have 3 angles, a,b, and c. Angle a shows relation of x and z axis Angle b shows relation of y and z axis Angle c ...
-4
votes
1answer
35 views

Orthogonal vectors and potential [on hold]

given the potential $ψ(x;y)$, such that $dψ=−u_2dx+u_1dy$, why are $∇ψ=(−u_2;u_1)$ and $ψ(x;y)=c$ orthogonal vectors ? $c \in \mathbb{R}$ is a constant, and $\mathbf{u}(x; y) = (u_1(x;y); u_2(x;y))$, ...
-1
votes
1answer
62 views

Proof inequality using Lagrange Multipliers

Is it possible: $a,b,c$ are non-negative real numbers for which holds that $a+b+c=3.$ Prove the following inequality: $$ 4\ge a^2b+b^2c+c^2a+abc $$ Is it possible using Lagrange Multipliers. I ...
0
votes
0answers
28 views

Cartesian product between manifolds

I was given the following exercise: Show that if $M$ is a $k$-manifold without boundary in $\mathbb{R}^m$, and if $N$ is an $l$-manifold in $\mathbb{R}^n$, then $M \times N$ is a $k+l$ manifold in ...