*Explicit numerical methods for stiff differential equations*

**DUMKA3** - integrates initial value problems for systems of first order
ordinary differential equations y'=f(y,t). It is based on a family of explicit
Runge-Kutta-Chebyshev formulas of order three. It uses optimal third order
accuracy stability polynomials with the largest stability region along the
negative real axis. The algorithm of construction of high order explicit
methods for stiff differential equations described in the article

*Alexei A. Medovikov "High order explicit
methods for parabolic equations" BIT1998, Vol. 38, No.2, pp.372-390*.

In this article the following idea has been formulated: how to "finish sequence
of steps to get method of the desired order": if you define a Runge-Kutta
method **A,** then one can define Runge-Kutta method **B**, so that
composition of two methods **C=B(A)** (method **B** used after method **A**)
is the method order p.

For example, described in this article is the algorithm of construction of
s-stage (number of RHS evaluations) and p-order explicit method is based on s-p
Euler steps (method **A**) and explicit p-stage remainder **B**, so that
composition of these two methods (method **C=B(A)**) becomes a method of
order p. To choose explicit Euler steps (method** A**) I take a p-order
optimal stability polynomial degree s, then I take out p roots from this
polynomial and the rest of roots made up sequence of Euler steps for method A,
the reminder method (B) finish construction of optimal stability polynomial, so
that method C has optimal stability polynomial I've chosen, and at the same
time lead to p-order composition method C=B(A). As the result I have p order
method (for non-liner problems) with optimal stability polynomial. The same
idea was used for construction fourth order explicit solvers dumka4 and rock4.

These formulas have been derived based on theory of composition methods. To
read about composition methods I would recommend the book of

*Ernst Hairer Syvert Paul
Nørsett Gerhard Wanner Solving Ordinary Differential Equations I. Nonstiff
Problems. Springer Series in Comput. Mathematics, Vol. 8, Springer-Verlag 1987,
Second revised edition 1993.** *

and

*Ernst Hairer Gerhard
Wanner Solving Ordinary Differential Equations II. Stiff and
Differential-Algebraic Problems. Springer Series in Comput. Mathematics, Vol.
14, Springer-Verlag 1991, Second revised edition 1996.*

Professors Ernst
Hairer and Gerhard
Wanner have contributed several ideas into dumka3, especially I would
recommend reading about embedded formulas, this algorithm allow efficient
step-size control with just a minor increasing of computational efforts.

Perhaps the major reason stable explicit methods have not been used for so long
is the problem with internal stability of these methods, which is closely
related to the similar problem for Chebyshev iterative methods: inspite of the
fact that Chebyshev polynomial is stable, computation of high degree Chebyshev
polynomial may be unstable, because intermediate polynomials can be large. This
problem was overcome by an algorithm proposed by Lebedev and Finogenov for
Chebyshev iterative methods, which is based on special sequence of roods of
Chebyshev polynomials. This "special sequence" forces intermediate
polynomials to be shifted Chebyshev polynomials, so that intermediate results
are obtained from stable polynomials as well. Ordering of parameters in DUMKA3
was done mostly by V.I. Lebedev and of course it would be interesting to read
articles of my scientific advisor - Professor V.I. Lebedev.

The problem of internal instability can be overcome by using of recurrent
relations for orthogonal polynomials. Initially this idea was formulated for
Chebyshev iterative methods, then it was used for explicit ODE solvers. For
more details please refer to articles of Professors van der Houwen, B.P. Sommeijer, L.F. Shampaine, J.G. Verwer.

Because of this wonderful idea and because of works of professor Lebedev on
orthogonal polynomials which approximate minimal polynomials with weight
functions, we come up with idea of using recurrent relations for orthogonal
polynomials different from Chebyshev polynomials, which approximate polynomials
of least deviation from zero with a weight. If we choose the weight to be also
a polynomial then the product of the weight and the polynomial is a polynomial,
but this product of the polynomial and weight function is close to a polynomial
of least deviation from zero which approximates the exponential function in the
vicinity of the origin used by dumka2, dumka3 and dumka4. So, we have
calculated weights for different degrees of almost optimal (but orthogonal)
stability polynomials and use reccurent relation to build final polynomial and
then multiply by weight. All intermediate stages of this reccurent process are
stable and as the result we do not have intermediate instability. For details I
would highly recommend to read the article

*Assyr Abdulle, Alexei A. Medovikov: Second order Chebyshev methods based on
orthogonal polynomials Numer. Math. 90 (2001) 1, 1-18*, and PhD
of Assyr Abdulle, where he developed these ideas for fourth order explicit
methods based on orthogonal polynomials (
Abdulle A: Fourth order Chebyshev methods with recurrence relation
SIAM J SCI COMPUT 23 (6): 2041-2054 MAY 17 2002).