Citation
Hagstrom, Thomas Michael (1983) Reduction of unbounded domains to bounded domains for partial differential equation problems. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd09062006104459
Abstract
Many boundary value problems which arise in applied mathematics are given in unbounded domains. Here we develop a theory for the imposition of boundary conditions at an artificial boundary which lead to finite domain problems that are equivalent to the unbounded domain problems from which they come. By considering the Cauchy problem with initial data in the appropriate space of functions on the artificial boundary, we show that satisfaction of the boundary conditions at infinity is equivalent to satisfaction of a certain projection condition, at the artificial boundary. This leads to an equivalent finite problem. The solvability of the finite problem is discussed and estimates of the solution in terms of the inhomogeneous data are given.
Applications of our reduction to problems whose coefficients are independent of the unbounded coordinate are considered first. For a class of problems we shall term 'separable', solutions in the tail can be developed in an eigenfunction expansion. These expansions are used to write down an explicit representation of the projection, which is useful in computations. Specific problems considered here include elliptic equations in cylindrical domains. Spatially unbounded parabolic and hyperbolic problems are also discussed. Here, the eigenfunction expansions must include continuous transform variables.
We use these 'constant tail' results to develop a perturbation theory for the case when the coefficients depend upon the unbounded coordinate. This theory is based on Duhamel's principle and is seen to be especially useful when the 'limiting' problem possesses an exponential dichotomy. We apply our results to the Helmholtz equation, perturbed hyperbolic systems and nonlinear problems. We present a numerical solution of the Bratu problem in a semiinfinite, twodimensional, stepped channel to illustrate our method.
Item Type:  Thesis (Dissertation (Ph.D.)) 

Degree Grantor:  California Institute of Technology 
Division:  Engineering and Applied Science 
Major Option:  Applied And Computational Mathematics 
Thesis Availability:  Public (worldwide access) 
Research Advisor(s): 

Thesis Committee: 

Defense Date:  24 May 1983 
Record Number:  CaltechETD:etd09062006104459 
Persistent URL:  http://resolver.caltech.edu/CaltechETD:etd09062006104459 
Default Usage Policy:  No commercial reproduction, distribution, display or performance rights in this work are provided. 
ID Code:  3356 
Collection:  CaltechTHESIS 
Deposited By:  Imported from ETDdb 
Deposited On:  22 Sep 2006 
Last Modified:  26 Dec 2012 02:59 
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