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Kalkulus |
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Dalam kalkulus, diferensial menyatakan perubahan suatu fungsi terhadap perubahan variabel bebas. Diferensial didefinisikan dengandengan menyatakan turunan terhadap , sedangkan menyatakan suatu peubah real tambahan; sehingga merupakan suatu fungsi dari dan ). Karena itu, notasi tersebut berlaku suatu persamaanPada persamaan ini, turunan dinyatakan dengan notasi Leibniz , dan bentuk notasi turunan menyerupai hasil bagi diferensial. Alternatif dari notasi ini juga ditulis sebagai
The precise meaning of the variables and depends on the context of the application and the required level of mathematical rigor. The domain of these variables may take on a particular geometrical significance if the differential is regarded as a particular differential form, or analytical significance if the differential is regarded as a linear approximation to the increment of a function. Traditionally, the variables and are considered to be very small (infinitesimal), and this interpretation is made rigorous in non-standard analysis.
Sejarah dan penggunaan
[sunting | sunting sumber]Diferensial pertama kali diperkenalkan melalui definisi intuitif atau heuristik oleh Isaac Newton. Gottfried Leibniz kemudian melanjutkan pengembangan konsep tersebut lebih lanjut dengan memandang diferensial sebagai perubahan dengan bilangan yang sangat kecil (infinitesimal) di nilai fungsi , yang bersesuaian dengan perubahan infinitesimal di argumen fungsi . Oleh karena itu, tingkat perubahan sesaat (instantaneous rate of change) terhadap , yang merupakan nilai dari turunan fungsi, ditulis menggunakan pecahanNotasi turunan ini dikenal dengan sebutan notasi Leibniz. Pada notasi tersebut, hasil bagi bukanlah suatu infinitesimal, melainkan suatu bilangan real.
Penggunaan infinteismal pada notasi tersebut telah dikritik secara luas, seperti Bishop Berkeley dalam pamflet terkenalnya yang berjudul The Analyst. Augustin-Louis Cauchy (1823) mendefinisikan diferensial tanpa membandingkan atomism of Leibniz's infinitesimals.[1][2] Instead, Cauchy, following d'Alembert, inverted the logical order of Leibniz and his successors: the derivative itself became the fundamental object, defined as a limit of difference quotients, and the differentials were then defined in terms of it. That is, one was free to define the differential by an expression
in which and are simply new variables taking finite real values,[3] not fixed infinitesimals as they had been for Leibniz.[4]
According to (Boyer 1959, hlm. 12), Cauchy's approach was a significant logical improvement over the infinitesimal approach of Leibniz because, instead of invoking the metaphysical notion of infinitesimals, the quantities and could now be manipulated in exactly the same manner as any other real quantities in a meaningful way. Cauchy's overall conceptual approach to differentials remains the standard one in modern analytical treatments,[5] although the final word on rigor, a fully modern notion of the limit, was ultimately due to Karl Weierstrass.[6]
In physical treatments, such as those applied to the theory of thermodynamics, the infinitesimal view still prevails. (Courant & John 1999, hlm. 184) reconcile the physical use of infinitesimal differentials with the mathematical impossibility of them as follows. The differentials represent finite non-zero values that are smaller than the degree of accuracy required for the particular purpose for which they are intended. Thus "physical infinitesimals" need not appeal to a corresponding mathematical infinitesimal in order to have a precise sense.
Following twentieth-century developments in mathematical analysis and differential geometry, it became clear that the notion of the differential of a function could be extended in a variety of ways. In real analysis, it is more desirable to deal directly with the differential as the principal part of the increment of a function. This leads directly to the notion that the differential of a function at a point is a linear functional of an increment . This approach allows the differential (as a linear map) to be developed for a variety of more sophisticated spaces, ultimately giving rise to such notions as the Fréchet or Gateaux derivative. Likewise, in differential geometry, the differential of a function at a point is a linear function of a tangent vector (an "infinitely small displacement"), which exhibits it as a kind of one-form: the exterior derivative of the function. In non-standard calculus, differentials are regarded as infinitesimals, which can themselves be put on a rigorous footing (see differential (infinitesimal)).
Definition
[sunting | sunting sumber]![](http://upload.wikimedia.org/wikipedia/commons/thumb/a/a9/Sentido_geometrico_del_diferencial_de_una_funcion.png/360px-Sentido_geometrico_del_diferencial_de_una_funcion.png)
Diferensial didefinisikan dalam perlakuan kalkulus diferensial yang modern.[7] Definisi mengatakan bahwa diferensial fungsi dari suatu peubah real merupakan fungsi dari dua peubah variabel bebas dan . Ini ditulis sebagai
Pada definisi di atas, seseorang dapat memandangnya sebagai , atau sederhananya, . Jika , maka diferensial dapat ditulis pula sebagai . Karena , maka secara konvensional ditulis sebagai sehingga berlaku persamaan berikut:
Gagasan tersebut dapat diterima dengan luas, terutama saat mencari hampiran linear dari suatu fungsi dengan nilai dari pertambahan yang cukup kecil. Lebih tepatnya, jika adalah suatu fungsi yang terdiferensialkan di , maka selisih dari nilai memenuhi bahwaCatatan bahwa galat pada hampiran tersebut memenuhi ketika . Dengan kata lain, identitas tersebut dihampiri sebagai dengan galatnya dapat dibuat sekecil mungkin dengan , yang dilakukan dengan cara memaksa nilai menjadi kecil, dalam artian,
ketika . Oleh karena demikian, diferensial fungsi dikenal sebagai bagian (linear) utama dalam pertambahan suatu fungsi: diferensial adalah fungsi linear dari pertambahan , dan walaupun galat tak linear, galat tersebut cenderung drastis menuju ke nol ketika menuju ke nol.
Diferensial dalam beberapa variabel
[sunting | sunting sumber]Operator / Fungsi | ||
---|---|---|
Diferensial | 1: | 2: |
Turunan parsial | ||
Turunan total |
Mengutip dari (Goursat 1904, I, §15), untuk fungsi dari variabel yang berjumlah lebih dari satu, turunan parsial dari terhadap salah satu variabel adalah bagian prinsip (principal part) dari perubahan di , yang dihasilkan dari perubahan di suatu variabel tersebut. Oleh karena itu, diferensial parsial melibatkan turunan parsial dari terhadap . Jumlah dari diferensial parsial terhadap semua variabel bebas sama dengan diferensial total yang merupakan bagian prinsip dari perubahan di yang dihasilkan dari perubahan di variabel bebas .
Diferensial parsial dapat dijelaskan lebih rinci lagi. Dalam kalkulus multivariabel, (Courant 1937b) mengutip bahwa jika adalah fungsi terdiferensialkan, maka berdasarkan definisi keterdiferensialan, pertambahan dengan suku galat cenderung menuju ke nol saat pertambahan sama-sama cenderung menuju ke nol. Oleh karena itu, diferensial total didefinisikan secara cermat sebagai
Karena, berdasarkan definisi,
maka
Untuk kasus diferensial parsial dengan satu variabel, maka berlaku identitas aproksimasi berikut Pada identitas tersebut, galat totalnya dapat dibuat sekecil-kecil mungkin dengan . Hal tersebut dilakukan dengan membatasi pertambahan menjadi bernilai kecil.
Penerapan diferensial total untuk mengestimasi galat
[sunting | sunting sumber]In measurement, the total differential is used in estimating the error of a function based on the errors of the parameters . Assuming that the interval is short enough for the change to be approximately linear:
and that all variables are independent, then for all variables,
This is because the derivative with respect to the particular parameter gives the sensitivity of the function to a change in , in particular the error . As they are assumed to be independent, the analysis describes the worst-case scenario. The absolute values of the component errors are used, because after simple computation, the derivative may have a negative sign. From this principle the error rules of summation, multiplication etc. are derived, e.g.:
- Let ;
- ; evaluating the derivatives
- Δf = bΔa + aΔb; dividing by f, which is a × b
- Δf/f = Δa/a + Δb/b
That is to say, in multiplication, the total relative error is the sum of the relative errors of the parameters.
To illustrate how this depends on the function considered, consider the case where the function is instead. Then, it can be computed that the error estimate is
- Δf/f = Δa/a + Δb/(b ln b)
with an extra 'ln b' factor not found in the case of a simple product. This additional factor tends to make the error smaller, as ln b is not as large as a bare b.
Diferemsial tingkat tinggi
[sunting | sunting sumber]Higher-order differentials of a function y = f(x) of a single variable x can be defined via:[8]
and, in general,
Informally, this motivates Leibniz's notation for higher-order derivatives
When the independent variable x itself is permitted to depend on other variables, then the expression becomes more complicated, as it must include also higher order differentials in x itself. Thus, for instance,
and so forth.
Similar considerations apply to defining higher order differentials of functions of several variables. For example, if f is a function of two variables x and y, then
where is a binomial coefficient. In more variables, an analogous expression holds, but with an appropriate multinomial expansion rather than binomial expansion.[9]
Higher order differentials in several variables also become more complicated when the independent variables are themselves allowed to depend on other variables. For instance, for a function f of x and y which are allowed to depend on auxiliary variables, one has
Because of this notational infelicity, the use of higher order differentials was roundly criticized by Hadamard 1935, who concluded:
- Enfin, que signifie ou que représente l'égalité
- A mon avis, rien du tout.
That is: Finally, what is meant, or represented, by the equality [...]? In my opinion, nothing at all. In spite of this skepticism, higher order differentials did emerge as an important tool in analysis.[10]
In these contexts, the nth order differential of the function f applied to an increment Δx is defined by
or an equivalent expression, such as
where is an nth forward difference with increment tΔx.
This definition makes sense as well if f is a function of several variables (for simplicity taken here as a vector argument). Then the nth differential defined in this way is a homogeneous function of degree n in the vector increment Δx. Furthermore, the Taylor series of f at the point x is given by
The higher order Gateaux derivative generalizes these considerations to infinite dimensional spaces.
Sifat-sifat
[sunting | sunting sumber]A number of properties of the differential follow in a straightforward manner from the corresponding properties of the derivative, partial derivative, and total derivative. These include:[11]
- Linearity: For constants a and b and differentiable functions f and g,
- Product rule: For two differentiable functions f and g,
An operation d with these two properties is known in abstract algebra as a derivation. They imply the Power rule
In addition, various forms of the chain rule hold, in increasing level of generality:[12]
- If y = f(u) is a differentiable function of the variable u and u = g(x) is a differentiable function of x, then
- If y = f(x1, ..., xn) and all of the variables x1, ..., xn depend on another variable t, then by the chain rule for partial derivatives, one has
- Heuristically, the chain rule for several variables can itself be understood by dividing through both sides of this equation by the infinitely small quantity dt.
- More general analogous expressions hold, in which the intermediate variables xi depend on more than one variable.
Perumusan umum
[sunting | sunting sumber]A consistent notion of differential can be developed for a function f : Rn → Rm between two Euclidean spaces. Let x,Δx ∈ Rn be a pair of Euclidean vectors. The increment in the function f is
If there exists an m × n matrix A such that
in which the vector ε → 0 as Δx → 0, then f is by definition differentiable at the point x. The matrix A is sometimes known as the Jacobian matrix, and the linear transformation that associates to the increment Δx ∈ Rn the vector AΔx ∈ Rm is, in this general setting, known as the differential df(x) of f at the point x. This is precisely the Fréchet derivative, and the same construction can be made to work for a function between any Banach spaces.
Another fruitful point of view is to define the differential directly as a kind of directional derivative:
which is the approach already taken for defining higher order differentials (and is most nearly the definition set forth by Cauchy). If t represents time and x position, then h represents a velocity instead of a displacement as we have heretofore regarded it. This yields yet another refinement of the notion of differential: that it should be a linear function of a kinematic velocity. The set of all velocities through a given point of space is known as the tangent space, and so df gives a linear function on the tangent space: a differential form. With this interpretation, the differential of f is known as the exterior derivative, and has broad application in differential geometry because the notion of velocities and the tangent space makes sense on any differentiable manifold. If, in addition, the output value of f also represents a position (in a Euclidean space), then a dimensional analysis confirms that the output value of df must be a velocity. If one treats the differential in this manner, then it is known as the pushforward since it "pushes" velocities from a source space into velocities in a target space.
Pendekatan lain
[sunting | sunting sumber]Although the notion of having an infinitesimal increment dx is not well-defined in modern mathematical analysis, a variety of techniques exist for defining the infinitesimal differential so that the differential of a function can be handled in a manner that does not clash with the Leibniz notation. These include:
- Defining the differential as a kind of differential form, specifically the exterior derivative of a function. The infinitesimal increments are then identified with vectors in the tangent space at a point. This approach is popular in differential geometry and related fields, because it readily generalizes to mappings between differentiable manifolds.
- Differentials as nilpotent elements of commutative rings. This approach is popular in algebraic geometry.[13]
- Differentials in smooth models of set theory. This approach is known as synthetic differential geometry or smooth infinitesimal analysis and is closely related to the algebraic geometric approach, except that ideas from topos theory are used to hide the mechanisms by which nilpotent infinitesimals are introduced.[14]
- Differentials as infinitesimals in hyperreal number systems, which are extensions of the real numbers which contain invertible infinitesimals and infinitely large numbers. This is the approach of nonstandard analysis pioneered by Abraham Robinson.[15]
Contoh dan penerapan
[sunting | sunting sumber]Differentials may be effectively used in numerical analysis to study the propagation of experimental errors in a calculation, and thus the overall numerical stability of a problem (Courant 1937a). Suppose that the variable x represents the outcome of an experiment and y is the result of a numerical computation applied to x. The question is to what extent errors in the measurement of x influence the outcome of the computation of y. If the x is known to within Δx of its true value, then Taylor's theorem gives the following estimate on the error Δy in the computation of y:
where ξ = x + θΔx for some 0 < θ < 1. If Δx is small, then the second order term is negligible, so that Δy is, for practical purposes, well-approximated by dy = f'(x)Δx.
The differential is often useful to rewrite a differential equation
in the form
in particular when one wants to separate the variables.
Catatan
[sunting | sunting sumber]- ^ For a detailed historical account of the differential, see Boyer 1959, especially page 275 for Cauchy's contribution on the subject. An abbreviated account appears in Kline 1972, Chapter 40.
- ^ Cauchy explicitly denied the possibility of actual infinitesimal and infinite quantities (Boyer 1959, hlm. 273–275), and took the radically different point of view that "a variable quantity becomes infinitely small when its numerical value decreases indefinitely in such a way as to converge to zero" (Cauchy 1823, hlm. 12; translation from Boyer 1959, hlm. 273).
- ^ Boyer 1959, hlm. 275
- ^ Boyer 1959, hlm. 12: "The differentials as thus defined are only new variables, and not fixed infinitesimals..."
- ^ Courant 1937a, II, §9: "Here we remark merely in passing that it is possible to use this approximate representation of the increment by the linear expression to construct a logically satisfactory definition of a "differential", as was done by Cauchy in particular."
- ^ Boyer 1959, hlm. 284
- ^ See, for instance, the influential treatises of Courant 1937a, Kline 1977, Goursat 1904, and Hardy 1908. Tertiary sources for this definition include also Tolstov 2001 and Itô 1993, §106.
- ^ Cauchy 1823. See also, for instance, Goursat 1904, I, §14.
- ^ Goursat 1904, I, §14
- ^ In particular to infinite dimensional holomorphy (Hille & Phillips 1974) and numerical analysis via the calculus of finite differences.
- ^ Goursat 1904, I, §17
- ^ Goursat 1904, I, §§14,16
- ^ Eisenbud & Harris 1998.
- ^ See Kock 2006 and Moerdijk & Reyes 1991.
- ^ See Robinson 1996 and Keisler 1986.
See also
[sunting | sunting sumber]References
[sunting | sunting sumber]- Boyer, Carl B. (1959), The history of the calculus and its conceptual development, New York: Dover Publications, MR 0124178.
- Cauchy, Augustin-Louis (1823), Résumé des Leçons données à l'Ecole royale polytechnique sur les applications du calcul infinitésimal, diarsipkan dari versi asli tanggal 2007-07-08, diakses tanggal 2009-08-19 .
- Courant, Richard (1937a), Differential and integral calculus. Vol. I, Wiley Classics Library, New York: John Wiley & Sons (dipublikasikan tanggal 1988), ISBN 978-0-471-60842-4, MR 1009558.
- Courant, Richard (1937b), Differential and integral calculus. Vol. II, Wiley Classics Library, New York: John Wiley & Sons (dipublikasikan tanggal 1988), ISBN 978-0-471-60840-0, MR 1009559.
- Courant, Richard; John, Fritz (1999), Introduction to Calculus and Analysis Volume 1, Classics in Mathematics, Berlin, New York: Springer-Verlag, ISBN 3-540-65058-X, MR 1746554
- Eisenbud, David; Harris, Joe (1998), The Geometry of Schemes, Springer-Verlag, ISBN 0-387-98637-5.
- Fréchet, Maurice (1925), "La notion de différentielle dans l'analyse générale", Annales Scientifiques de l'École Normale Supérieure, Série 3, 42: 293–323, doi:10.24033/asens.766, ISSN 0012-9593, MR 1509268.
- Goursat, Édouard (1904), A course in mathematical analysis: Vol 1: Derivatives and differentials, definite integrals, expansion in series, applications to geometry, E. R. Hedrick, New York: Dover Publications (dipublikasikan tanggal 1959), MR 0106155.
- Hadamard, Jacques (1935), "La notion de différentiel dans l'enseignement", Mathematical Gazette, XIX (236): 341–342, doi:10.2307/3606323, JSTOR 3606323.
- Hardy, Godfrey Harold (1908), A Course of Pure Mathematics, Cambridge University Press, ISBN 978-0-521-09227-2.
- Hille, Einar; Phillips, Ralph S. (1974), Functional analysis and semi-groups, Providence, R.I.: American Mathematical Society, MR 0423094.
- Itô, Kiyosi (1993), Encyclopedic Dictionary of Mathematics (edisi ke-2nd), MIT Press, ISBN 978-0-262-59020-4.
- Kline, Morris (1977), "Chapter 13: Differentials and the law of the mean", Calculus: An intuitive and physical approach, John Wiley and Sons.
- Kline, Morris (1972), Mathematical thought from ancient to modern times
(edisi ke-3rd), Oxford University Press (dipublikasikan tanggal 1990), ISBN 978-0-19-506136-9
- Keisler, H. Jerome (1986), Elementary Calculus: An Infinitesimal Approach (edisi ke-2nd).
- Kock, Anders (2006), Synthetic Differential Geometry (PDF) (edisi ke-2nd), Cambridge University Press.
- Moerdijk, I.; Reyes, G.E. (1991), Models for Smooth Infinitesimal Analysis, Springer-Verlag.
- Robinson, Abraham (1996), Non-standard analysis, Princeton University Press, ISBN 978-0-691-04490-3.
- Tolstov, G.P. (2001) [1994], "Differential", dalam Hazewinkel, Michiel, Encyclopedia of Mathematics, Springer Science+Business Media B.V. / Kluwer Academic Publishers, ISBN 978-1-55608-010-4.
External links
[sunting | sunting sumber]- Differential Of A Function at Wolfram Demonstrations Project