## Abstract

The nonlinear two-point boundary value problem (TPBVP for short) \[u_{xx}+u^{3}=0,\quad u(0)=u(1)=0,\] offers several insights into spectral methods.

First, it has been proved a priori that \[\int u(x)dx=\frac p{\sqrt{2}}.\] By building this constraint into the spectral approximation, the accuracy of \(N=1\) degrees of freedom is achieved from the work of solving a system with only N degrees of freedom. When N is small, generic polynomial system solvers, such as those in the computer algebra system Maple, can find all roots of the polynomial system, such as a spectral discretization of the TPBVP.

Our second point is that floating point arithmetic in lieu of exact arithmetic can double the largest practical value of N. (Rational numbers with a huge number of digits are avoided, and eliminating M symbols like \(\sqrt{2}\) and p reduces \(N+M\)-variate polynomials to polynomials in just the N unknowns.) Third, a disadvantage of an “all roots” approach is that the polynomial solver generates many roots \(( 3^N-1)\) -for our example – which are genuine solutions to the \(N\)-term discretization but spurious in the sense that they are not close to the spectral coefficients of a true solution to the TPBVP.

We show here that a good tool for “root-exclusion” is calculating \[\rho=\sqrt{\sum\limits_{n=1}^{N}b_{n}^{2}};\] spurious roots have \(\rho\) larger than that for the physical solution by at least an order of magnitude. The \(\rho\)-criterion is suggestive rather than infallible, but root exclusion is very hard, and the best approach is to apply multiple tools with complementary failings.

## Authors

John P. **Boyd**

(Department of Climate & Space Sciences and Engineering, University of Michigan, United States)

Calin-Ioan **Gheorghiu**

(Tiberiu Popoviciu Institute of Numerical Analysis, Romanian Academy)

## Keywords

### References

See the expanding block below.

## Cite this paper as

J.P. Boyd, C.I. Gheorghiu, *All roots spectral methods: Constraints, floating point arithmetic and root exclusion*, Applied Mathematics Letters 67 (2017) 28–32

DOI: 10.1016/j.aml.2016.11.015

?

## About this paper

##### Print ISSN

?

##### Online ISSN

?

##### Google Scholar Profile

?

[1] J.P. Boyd, Tracing multiple solution branches for nonlinear ordinary differential equations: Chebyshev and Fourier spectral methods and a degree-increasing spectral homotopy [DISH], J. Sci. Comput. 19 (2016) 1113–1143.

[2] J.P. Boyd, Degree-increasing [N to N + 1] homotopy for Chebyshev and Fourier spectral methods, Appl. Math. Lett. 57 (2016) 77–81.

[3] C.I. Gheorghiu, D. Trif, The numerical approximation to positive solution for some reaction–diffusion problems, Pure Math. Appl.: Math. Optim. 11 (2001) 243–253.

[4] J.P. Boyd, Chebyshev and Fourier Spectral Methods, Dover, New York, 2001.

[5] C.-I. Gheorghiu, Spectral Methods for Differential Problems, Casa Cartii de Stiinta, Cluj-Napoca, Romania, 2007, 157 pp.

Available at http://www.ictp.acad.ro/gheorghiu/spectral.pdf.

[6] B.A. Finlayson, The Method of Weighted Residuals and Variational Principles, second ed., SIAM, New York, 2013.

[7] J.P. Boyd, Chebyshev and Legendre spectral methods in algebraic manipulation languages, J. Symbolic Comput. 16 (1993) 377–399.

[8] J.P. Boyd, Correcting three errors in Kantorovich & Krylov’s approximate methods of higher analysis: Energizing perturbation series and Chebyshev and legendre spectral algorithms with computer algebra, Amer. Math. Monthly 123 (2016) 241–257.