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The finite difference is the discrete analog of the derivative. The finite forward difference of a function is defined as
(1) |
and the finite backward difference as
(2) |
The forward finite difference is implemented in the Wolfram Language as DifferenceDelta[f, i].
If the values are tabulated at spacings , then the notation
(3) |
is used. The th forward difference would then be written as , and similarly, the th backward difference as .
However, when is viewed as a discretization of the continuous function , then the finite difference is sometimes written
(4) |
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(5) |
where denotes convolution and is the odd impulse pair. The finite difference operator can therefore be written
(6) |
An th power has a constant th finite difference. For example, take and make a difference table,
(7) |
The column is the constant 6.
Finite difference formulas can be very useful for extrapolating a finite amount of data in an attempt to find the general term. Specifically, if a function is known at only a few discrete values , 1, 2, ... and it is desired to determine the analytical form of , the following procedure can be used if is assumed to be a polynomial function. Denote the th value in the sequence of interest by . Then define as the forward difference , as the second forward difference , etc., constructing a table as follows
(8) |
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(9) |
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(10) |
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(11) |
Continue computing , , etc., until a 0 value is obtained. Then the polynomial function giving the values is given by
(12) |
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(13) |
When the notation , , etc., is used, this beautiful equation is called Newton's forward difference formula. To see a particular example, consider a sequence with first few values of 1, 19, 143, 607, 1789, 4211, and 8539. The difference table is then given by
(14) |
Reading off the first number in each row gives , , , , . Plugging these in gives the equation
(15) |
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(16) |
which indeed fits the original data exactly.
Formulas for the derivatives are given by
(17) |
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(18) |
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(19) |
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(20) |
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(21) |
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(22) |
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(23) |
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(24) |
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(25) |
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(27) |
(Beyer 1987, pp. 449-451; Zwillinger 1995, p. 705).
Formulas for integrals of finite differences
(28) |
are given by Beyer (1987, pp. 455-456).
Finite differences lead to difference equations, finite analogs of differential equations. In fact, umbral calculus displays many elegant analogs of well-known identities for continuous functions. Common finite difference schemes for partial differential equations include the so-called Crank-Nicolson, Du Fort-Frankel, and Laasonen methods.
REFERENCES:
Abramowitz, M. and Stegun, I. A. (Eds.). "Differences." §25.1 in Handbook of Mathematical Functions with Formulas, Graphs, and Mathematical Tables, 9th printing. New York: Dover, pp. 877-878, 1972.
Beyer, W. H. CRC Standard Mathematical Tables, 28th ed. Boca Raton, FL: CRC Press, pp. 429-515, 1987.
Boole, G. and Moulton, J. F. A Treatise on the Calculus of Finite Differences, 2nd rev. ed. New York: Dover, 1960.
Conway, J. H. and Guy, R. K. "Newton's Useful Little Formula." In The Book of Numbers. New York: Springer-Verlag, pp. 81-83, 1996.
Fornberg, B. "Calculation of Weights in Finite Difference Formulas." SIAM Rev. 40, 685-691, 1998.
Iyanaga, S. and Kawada, Y. (Eds.). "Interpolation." Appendix A, Table 21 in Encyclopedic Dictionary of Mathematics. Cambridge, MA: MIT Press, pp. 1482-1483, 1980.
Jordan, C. Calculus of Finite Differences, 3rd ed. New York: Chelsea, 1965.
Levy, H. and Lessman, F. Finite Difference Equations. New York: Dover, 1992.
Milne-Thomson, L. M. The Calculus of Finite Differences. London: Macmillan, 1951.
Richardson, C. H. An Introduction to the Calculus of Finite Differences. New York: Van Nostrand, 1954.
Spiegel, M. Calculus of Finite Differences and Differential Equations. New York: McGraw-Hill, 1971.
Stirling, J. Methodus differentialis, sive tractatus de summation et interpolation serierum infinitarium. London, 1730. English translation by Holliday, J. The Differential Method: A Treatise of the Summation and Interpolation of Infinite Series. 1749.
Tweddle, C. James Stirling: A Sketch of His Life and Works Along with his Scientific Correspondence. Oxford, England: Oxford University Press, pp. 30-45, 1922.
Weisstein, E. W. "Books about Finite Difference Equations." http://www.ericweisstein.com/encyclopedias/books/FiniteDifferenceEquations.html.
Zwillinger, D. (Ed.). "Difference Equations" and "Numerical Differentiation." §3.9 and 8.3.2 in CRC Standard Mathematical Tables and Formulae. Boca Raton, FL: CRC Press, pp. 228-235 and 705-705, 1995.
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