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MATH 348 HW
Due Monday March 14 on BrightSpace by midnight 1. (10 points) Write a Python function named GaussLegendre to calculate Gauss-Legendre integration ˆ 1 −1 f(x)dx = n∑ i=1 wif(xi) where {xi}ni=1 are the roots of the the degree-n Legendre polynomial Ln, which satisfied the recursive rela- tionship (n+ 1)Ln+1(x) = (2n+ 1)xLn(x)− nLn−1(x) for n = 1, 2, 3, . . ., with L−1(x) = 0 and L0(x) = 1. • Arguments: – f: the integrand, a function – n: the degree of the Legendre polynomial to compute xi, an integer. • Return the Gauss-Legendre approximation of the integral, a float value. 2. (10 points) Extend the method to evaluate ˆ b a g(x)dx. Note that the Legendre polynomials are orthogonal on the integral [−1, 1], so a change of variuable is needed. (a) (5 points) Write a Python function named GaussLegendre2 (you may use the GaussLegendre function in Question 1). • Arguments: – g: the integrand, a function – a, b: the integral limits, float values – n: the degree of the Legendre polynomial to compute xi, an integer. • Return the Gauss-Legendre approximation of the integral, a float value. (b) (5 points) Use the function GaussLegendre2 to evaluate ˆ pi/2 −pi/2 cosxdx, with n = 6, and compute the error (the true value is 2). Find the step size h needed for the Simpson’s method to reach the same error. 3. (10 points) Suppose the function f(x) is smooth enough. (a) (5 points) Theoretically estimate the error of Gauss-Legendre integral ´ b a f(x)dx for a given n. (b) (5 points) Compare with the results in Question 2(b).