How did my professor get from here to here? DiffEq

In summary, the conversation discusses the process of solving a homogeneous PDE with boundary conditions, using separation of variables. The goal is to find non-trivial solutions, which requires keeping constants that are not equal to zero. The values of these constants are determined by choosing specific values of the variable k.
  • #1
grandpa2390
474
14

Homework Statement


X(x)=(Ae^kx+Be^-kx)
Y(y)=(Csin(ky)+D(cos(ky))
V(x,y)=(Ae^kx+Be^-kx)(Csin(ky)+D(cos(ky))

Homework Equations


separation of variables

The Attempt at a Solution


so our boundary condition says that as x->infinity , V=0
this is only possible if A=0

so A=0 and Be^-kx= 0
so X= Be^-kx
and
V(x,y)=(e^-kx)(Csin(ky)+D(cos(ky))

why is there an e^-kx on the front. Shouldn't there also be a B?
 
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  • #2
B can be "absorbed" by the arbitrary constants C and D.
 
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  • #3
TSny said:
B can be "absorbed" by the arbitrary constants C and D.
ah! you're right. distribute the B into the the parenthesis. I knew it was something so simple otherwise he wouldn't have glossed over it. lol.
Thanks!

just one last question. so B^e^-kx we keep because the constant is not equal to zero?
because the next part we solve for Y, and D ends up being 0 but we keep Csin(ky) which equals 0 because sin(0)=0. so we keep that one but eliminate the part where D=0.

so we keep the part where the constant is not equal to zero?
 
  • #4
grandpa2390 said:
just one last question. so B^e^-kx we keep because the constant is not equal to zero?
because the next part we solve for Y, and D ends up being 0 but we keep Csin(ky) which equals 0 because sin(0)=0. so we keep that one but eliminate the part where D=0.

so we keep the part where the constant is not equal to zero?
Yes. If a boundary condition forces a constant factor of a term to be zero, then you drop that term. Otherwise, keep the term.
 
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  • #5
Apparently you are solving a homogeneous PDE with boundary conditions. The identically zero function is always a solution, but you want non-trivial solutions. So you hope not to have to take all the constants be zero. So you probably have something like ##Ce^{-kx}\sin(ky)## and you get your last boundary condition to be zero by choosing particular values of ##k## so the sine term can be zero without making ##C=0##. These values of ##k## determine the eigenvalues of the problem and allow nontrivial solutions.
 
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1. How does my professor solve differential equations?

There are various methods for solving differential equations, including separation of variables, substitution, and undetermined coefficients. Your professor likely has a preferred method or set of methods that they teach and use in their research.

2. How did my professor become an expert in differential equations?

Becoming an expert in any field, including differential equations, takes years of studying, practice, and dedication. Your professor likely pursued higher education, conducted research, and gained experience in the field to become an expert in differential equations.

3. What inspired my professor to study differential equations?

Every scientist has their own unique inspiration and reasons for studying a particular subject. Your professor may have been fascinated by the applications of differential equations, or they may have been drawn to the challenge and complexity of the subject.

4. How does my professor use differential equations in their research?

Differential equations are used in a wide range of scientific fields, including physics, engineering, and biology. Your professor may use differential equations to model and predict real-world phenomena in their specific area of research.

5. What advice does my professor have for understanding and solving differential equations?

Your professor likely has a wealth of knowledge and experience in solving differential equations. They may recommend practicing regularly, seeking help from classmates or tutors, and familiarizing yourself with different methods and techniques for solving these equations.

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