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Section 6.1 Cauchy-Euler Equation

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THE CAUCHY-EULER EQUATION Any linear differential equation of the from where a n,..., a 0 are constants, is said to be a Cauchy-Euler equation, or equidimensional equation. NOTE: The powers of x match the order of the derivative.

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We will confine our attention to solving the homogeneous second-order equation. The solution of higher-order equations follows analogously. The nonhomogeneous equation can be solved by variation of parameters once the complimentary function, y c (x), is found. HOMOGENEOUS 2 ND -ORDER CAUCHY-EULER EQUATION

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THE SOLUTION We try a solution of the form y = x m, where m must be determined. The first and second derivatives are, respectively, y′ = mx m − 1 and y″ = m(m − 1)x m − 2. Substituting into the differential equation, we find that x m is a solution of the equation whenever m is a solution of the auxiliary equation

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THREE CASES There are three cases to consider, depending on the roots of the auxiliary equation. Case I:Distinct Real Roots Case II:Repeated Real Roots Case III:Conjugate Complex Roots

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Let m 1 and m 2 denote the real roots of the auxiliary equation where m 1 ≠ m 2. Then form a fundamental solution set, and the general solution is CASE I — DISTINCT REAL ROOTS

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Let m 1 = m 2 denote the real root of the auxiliary equation. Then we obtain one solution—namely, Using the formula from Section 4.2, we obtain the solution Then the general solution is CASE II — REPEATED REAL ROOTS

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Let m 1 = α + βi and m 2 = α − βi, then the solution is Using Euler’s formula, we can obtain the fundamental set of solutions y 1 = x α cos(β ln x) and y 2 = x α sin(β ln x). Then the general solution is CASE III — CONJUGATE COMPLEX ROOTS

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ALTERNATE METHOD OF SOLUTION Any Cauchy-Euler equation can be reduced to an equation with constant coefficients by means of the substitution x = e t.

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HOMEWORK 1–45 odd

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