The Calculus of Proportional Alpha-Derivatives
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Rose-Hulman Undergraduate Mathematics Journal Volume 18 Issue 1 Article 2 The Calculus of Proportional alpha-Derivatives Laura A. LeGare Concordia College Grace K. Bryan Concordia College Follow this and additional works at: https://scholar.rose-hulman.edu/rhumj Recommended Citation LeGare, Laura A. and Bryan, Grace K. (2017) "The Calculus of Proportional alpha-Derivatives," Rose- Hulman Undergraduate Mathematics Journal: Vol. 18 : Iss. 1 , Article 2. Available at: https://scholar.rose-hulman.edu/rhumj/vol18/iss1/2 Rose- Hulman Undergraduate Mathematics Journal the calculus of proportional α-derivatives Laura A. LeGarea Grace K. Bryanb Volume 18, No. 1, Spring 2017 Sponsored by Rose-Hulman Institute of Technology Department of Mathematics Terre Haute, IN 47803 a [email protected] Concordia College, Moorhead, MN b scholar.rose-hulman.edu/rhumj Concordia College, Moorhead, MN Rose-Hulman Undergraduate Mathematics Journal Volume 18, No. 1, Spring 2017 the calculus of proportional α-derivatives Laura A. LeGare Grace K. Bryan Abstract.We introduce a new proportional α-derivative with parameter α 2 [0; 1], explore its calculus properties, and give several examples of our results. We begin with an introduction to our proportional α-derivative and some of its basic calculus properties. We next investigate the system of α-lines which make up our curved yet Euclidean geometry, as well as address traditional calculus concepts such as Rolle's Theorem and the Mean Value Theorem in terms of our α-derivative. We also introduce a new α-integral to be paired with our α-derivative, which leads to proofs of the Fundamental Theorem of Calculus Parts I and II, as applied to our formulas. Finally, we provide instructions on how to locate α-maximum and α- minimum values as they are related to our type of Euclidean geometry, including an increasing and decreasing test, concavity test, and first and second α-derivative tests. Acknowledgements: This research was supported by an NSF STEP grant (DUE 0969568) and by a gift to the Division of Sciences and Mathematics at Concordia College made in support of undergraduate research. We thank Professor Douglas R. Anderson for his guidance and support. RHIT Undergrad. Math. J., Vol. 18, No. 1 Page 21 1 Introduction When thinking of derivatives of the function f(t) in traditional calculus, it is common practice df d2f d3f to take the first derivative, dt , the second derivative, dt2 , the third derivative dt3 , and so on. Here, we propose a type of proportional derivative Dαf(t), with α 2 [0; 1], to explore the behavior of a function f(t) as it fluctuates between its identity and its first derivative. We begin with some essential definitions pertaining to our topic of proportional α- derivatives. Definition 1 (Conformable Differential Operator). Let α 2 [0; 1]. A differential operator Dα is conformable if and only if D0 is the identity operator and D1 is the classical differential operator. Specifically, Dα is conformable if and only if for a differentiable function f(t), d D0f(t) = f(t) and D1f(t) = f(t) = f 0(t): dt Remark 2. In this paper we will investigate a specific type of derivative that is motivated by the idea of a proportional-derivative (PD) controller from control theory, such as that used by Piltan, Mehrara, Meigolinedjad, and Bayat [1, Equations (4) and (5)], for example. PD controllers are used to tune the measured signal, say in a robot manipulator, toward the desired input signal. In particular, if U is the controller output over time, then 0 Uα = KVα e + Kpα e; where e(t) = θtarget(t) − θd(t) is the error between the measured and desired signals, and the tuning parameters KVα and Kpα stand for the derivative gain and the proportional gain, respectively. We now define the main object of interest in our paper. Definition 3 (A Type of Conformable Derivative). Let α 2 [0; 1]. The following differential operator Dα, defined via Dαf(t) = αf 0(t) + (1 − α)f(t) (1) is the α-derivative of f(t), and is conformable provided the function f(t) is differentiable at 0 d t and f (t) := dt f(t). Furthermore, the partial α-derivative with respect to t is given by @ Dαf(t; s) = α f(t; s) + (1 − α)f(t; s): t @t This definition is a special case of a more theoretical definition given in Anderson and Ulness [2, Definition 1.3]. Remark 4. We note that another definition of a conformable derivative appears in Camrud [3]. Page 22 RHIT Undergrad. Math. J., Vol. 18, No. 1 2 n Example 5. Let α 2 [0; 1]. If we take the α-derivative of a function f(t) = t at α = 8 , 0 ≤ n ≤ 8, n 2 Z, we have the following: α = 0 : D0f(t) = t2 1 1 2 α = : D 8 f(t) = :25t + :875t 8 2 2 2 α = : D 8 f(t) = :5t + :75t 8 3 3 2 α = : D 8 f(t) = :75t + :625t 8 4 4 2 α = : D 8 f(t) = t + :5t 8 5 5 2 α = : D 8 f(t) = 1:25t + :375t 8 6 6 2 α = : D 8 f(t) = 1:5t + :25t 8 7 7 2 α = : D 8 f(t) = 1:75t + :125t 8 α = 1 : D1f(t) = 2t: Graphically, these look like 8 6 t2 4 2 -2 -1 1 2 3 -2 -4 2t Figure 1: In this graph, we can see that as α increases from 0 to 1, the α-derivative of t2 bends from t2 toward 2t. Throughout the rest of the paper, we will be exploring the properties of this proportional α-derivative by investigating its implications with laws of traditional calculus, such as basic derivative and integration rules, as well as evaluating function behavior through local max- imums and minimums, increasing and decreasing tests, and concavity tests. In particular, RHIT Undergrad. Math. J., Vol. 18, No. 1 Page 23 in Section 2 we cover basic α-derivative rules, what a \horizontal line" looks like according to our proportional derivative, the definitions of both an α-secant line and an α-tangent line, as well as proofs of Rolle's Theorem and the Mean Value Theorem, followed by several examples. In Section 3, we provide a definition for an α-integral to be paired with our α- derivative, a proof of the Fundamental Theorem of Calculus with respect to our α-integral and α-derivative, and additional examples. Finally, in Section 4, we state definitions of α-increasing and α-decreasing, a critical point, and α-maximums and α-minimums, as well as proving theorems relating to an increasing/decreasing test, first and second α-derivative tests, and a concavity test, ending with a final example and theorems using the α-derivative to locate critical points in certain functions. 2 Lines and the MVT of Proportional α-Derivatives Using formula (1), in this section we cover basic α-derivative rules, what a \horizontal line" looks like according to our proportional derivative, the definitions of both an α-secant line and an α-tangent line, as well as proofs of Rolle's Theorem and the Mean Value Theorem, followed by several examples. We begin with the following basic α-derivative formulas, which are special cases of those found in Anderson and Ulness [2], but are included here for completeness: Lemma 6 (Basic α-Derivatives). Let the conformable differential operator Dα be given as in (1), where α 2 [0; 1]. Assume the functions f and g are differentiable as needed. Then (i) Dα[af + bg] = aDα[f] + bDα[g] for all a; b 2 R; (ii) Dαc = c(1 − α) for all constants c 2 R; (iii) Dα[fg] = fDα[g] + gDα[f] − (1 − α)[fg]; α gDα[f]−fDα[g] f (iv) D [f=g] = g2 + (1 − α) g . Proof. The proofs of (i) and (ii) follow immediately from Definition 3. We prove (iii), the α-Product Rule. Let the conditions given in Lemma 6 be satisfied. By traditional calculus, [fg]0 = fg0 + f 0g: Applying the α-derivative rather than the traditional derivative, we get Dα[fg] = α[fg]0 + (1 − α)[fg] = α[fg0 + f 0g] + (1 − α)[fg] = α[fg0] + α[f 0g] + (1 − α)[fg] = f[αg0 + (1 − α)g] + [αf 0 + (1 − α)f]g − (1 − α)[fg] = fDα[g] + gDα[f] − (1 − α)[fg]: This completes the proof of (iii). A similar process is used in proving (iv), the α-Quotient Rule. Page 24 RHIT Undergrad. Math. J., Vol. 18, No. 1 We now present a vital definition which establishes a type of exponential function for the α-derivative (1). Note that this is a special case of the exponential function introduced in Anderson and Ulness [2]. However, the results below that follow from the definition are new. Definition 7 (Conformable Exponential Function). Let α 2 (0; 1], the points s; t 2 R, and let the function p :[s; t] ! R be continuous. Then the conformable exponential function with respect to Dα in (1) is defined to be R t p(τ)−(1−α) dτ −( 1−α )(t−s) ep(t; s) := e s α ; e0(t; s) = e α ; (2) and, for a fixed value of s, satisfies α α Dt ep(t; s) = p(t)ep(t; s);Dt e0(t; s) = 0: (3) The following useful properties of the exponential function are a direct result of (2). Lemma 8 (Exponential Function Properties). Let p; q be continuous functions, and let t; s; r 2 R. For the conformable exponential function given in (2), the following properties hold: (i) ep(t; t) ≡ 1; (ii) ep(t; s)ep(s; r) = ep(t; r); (iii) 1 = e (s; t); ep(t;s) p (iv) e(1−α)(t; s) ≡ 1; 2 (v) e(α−1)(t; s) = e0(t; s); (vi) ep(t; s)eq(t; s) = e(p+q−1+α))(t; s); (vii) ep(t;s) = e (t; s).