Transcendental equation

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John Herschel, Description of a machine for resolving by inspection certain important forms of transcendental equations, 1832 Herschel - Description of a machine for resolving by inspection certain important forms of transcendental equations, 1832 - 687143.tiff
John Herschel, Description of a machine for resolving by inspection certain important forms of transcendental equations, 1832

In applied mathematics, a transcendental equation is an equation over the real (or complex) numbers that is not algebraic, that is, if at least one of its sides describes a transcendental function. [1] Examples include:

Contents

A transcendental equation need not be an equation between elementary functions, although most published examples are.

In some cases, a transcendental equation can be solved by transforming it into an equivalent algebraic equation. Some such transformations are sketched below; computer algebra systems may provide more elaborated transformations. [a]

In general, however, only approximate solutions can be found. [2]

Transformation into an algebraic equation

Ad hoc methods exist for some classes of transcendental equations in one variable to transform them into algebraic equations which then might be solved.

Exponential equations

If the unknown, say x, occurs only in exponents:

transforms to , which simplifies to , which has the solutions
This will not work if addition occurs "at the base line", as in
transforms, using y=2x, to which has the solutions , hence is the only real solution. [4]
This will not work if squares or higher power of x occurs in an exponent, or if the "base constants" do not "share" a common q.
transforms to which has the solutions hence , where and denote the real-valued branches of the multivalued function.

Logarithmic equations

If the unknown x occurs only in arguments of a logarithm function:

transforms, using exponentiation to base to which has the solutions If only real numbers are considered, is not a solution, as it leads to a non-real subexpression in the given equation.
This requires the original equation to consist of integer-coefficient linear combinations of logarithms w.r.t. a unique base, and the logarithm arguments to be polynomials in x. [5]
transforms, using to which is algebraic and has the single solution . [b] After that, applying inverse operations to the substitution equation yields

Trigonometric equations

If the unknown x occurs only as argument of trigonometric functions:

transforms to , and, after substitution, to which is algebraic [c] and can be solved. After that, applying obtains the solutions.

Hyperbolic equations

If the unknown x occurs only in linear expressions inside arguments of hyperbolic functions,

unfolds to which transforms to the equation which is algebraic [d] and can be solved. Applying obtains the solutions of the original equation.


Approximate solutions

Graphical solution of sin(x)=ln(x) Sin x = ln x svg.svg
Graphical solution of sin(x)=ln(x)

Approximate numerical solutions to transcendental equations can be found using numerical, analytical approximations, or graphical methods.

These equations can be solved by direct iteration by reordering the equation into the form and making an initial guess , computing which becomes and substituting it back into , etc. Convergence may be very slow. Some reorderings may diverge, so some other reordering that converges must be found. must be continuous and "sufficiently smooth" or the method may fail.

Numerical methods for solving arbitrary equations are called root-finding algorithms. By rearranging the equation into the form , if is continuous and differentiable, Newton's method involving taking the derivative of , is a common iterative method of approximating a root; an initial guess must be "sufficiently close" to the root of interest to converge to it.

In some cases, the equation can be well approximated using Taylor series near the zero. For example, for , the solutions of are approximately those of , namely and .

For a graphical solution, one method is to set each side of a single-variable transcendental equation equal to a dependent variable and plot the two graphs, using their intersecting points to find solutions (see picture).

Other solutions

See also

Notes

  1. For example, according to the Wolfram Mathematica tutorial page on equation solving, both and can be solved by symbolic expressions, while can only be solved approximatively.
  2. Squaring both sides obtains which has the additional solution ; however, the latter does not solve the unsquared equation.
  3. over an appropriate field, containing and
  4. over an appropriate field, containing

References

  1. I.N. Bronstein and K.A. Semendjajew and G. Musiol and H. Mühlig (2005). Taschenbuch der Mathematik (in German). Frankfurt/Main: Harri Deutsch. Here: Sect.1.6.4.1, p.45. The domain of equations is left implicit throughout the book.
  2. Bronstein et al., p.45-46
  3. Bronstein et al., Sect.1.6.4.2.a, p.46
  4. Bronstein et al., Sect.1.6.4.2.b, p.46
  5. Bronstein et al., Sect.1.6.4.3.b, p.46
  6. Bronstein et al., Sect.1.6.4.3.a, p.46
  7. Bronstein et al., Sect.1.6.4.4, p.46-47
  8. Bronstein et al., Sect.1.6.4.5, p.47
  9. V. A. Varyuhin, S. A. Kas'yanyuk, “On a certain method for solving nonlinear systems of a special type”, Zh. Vychisl. Mat. Mat. Fiz., 6:2 (1966), 347–352; U.S.S.R. Comput. Math. Math. Phys., 6:2 (1966), 214–221
  10. V.A. Varyukhin, Fundamental Theory of Multichannel Analysis (VA PVO SV, Kyiv, 1993) [in Russian]