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# SJUT/MAT210/Interpolation/Splines 2013-14S2

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Lecture slides based on Autar Kaw's Numerical Methods text, Chapter 5.05, found at http://nm.mathforcollege.com

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### SJUT/MAT210/Interpolation/Splines 2013-14S2

1. 1. St. John's University of Tanzania MAT210 NUMERICAL ANALYSIS 2013/14 Semester II INTERPOLATION Splines Kaw, Chapter 5.05
2. 2. MAT210 2013/14 Sem II 2 of 20 ● Direct, Newton Divided Difference & Lagrangian Interpolation ● Two approaches for finding the same nth order polynomial fit for all points in an data set ● Is splines just another way to do the same ● NO! ● It is Piecewise polynomial interpolation ● Each piece can be linear, quadratic or cubic Introduction
3. 3. MAT210 2013/14 Sem II 3 of 20 How do we avoid this?
4. 4. MAT210 2013/14 Sem II 4 of 20 By observing ● This function has distinct regions ● The interval from x ≈ -1 to -0.5 ● The interval from x ≈ -0.5 to -0.1 ● The interval from x ≈ -0.1 to +0.1 ● The interval from x ≈ 0.1 to 0.5 ● The interval from x ≈ 0.5 to 1 Though there is some symmetry... ● It would be better to fit different functions to different intervals
5. 5. MAT210 2013/14 Sem II 5 of 20 Piecewise Polynomials Rather than interpolating n+1 points with a single polynomial of degree n, put different polynomials on each interval S(x)= { s0 (x) , x∈[x0 ,x1) s1 (x) , x∈[x1, x2) ⋮ sn−1 (x) , x∈[xn−1 , xn]}where the sj are polynomials of (usually) small degree
6. 6. MAT210 2013/14 Sem II 6 of 20 Interpretation ● Piecewise linear = connect the dots ● Piecewise quadratic = parabolas between the dots ● But wait ● Two points uniquely define a line – linear is understandable ● Three points are needed for a parabola – How is the other degree of freedom set?
7. 7. MAT210 2013/14 Sem II 7 of 20 Splines ● In the connect the dots linear case, the curve is not “smooth” ● Add “smoothness” into the requirement ● Draftsmen achieved this smoothness with splines - a flexible strip of metal or wood
8. 8. MAT210 2013/14 Sem II 8 of 20 Splines ● In the connect the dots linear case, the curve is not “smooth” ● Add “smoothness” into the requirement ● Draftsmen achieved this smoothness with splines - a flexible strip of metal or wood ● Mathematicians achieve it by matching derivatives at the end points of the intervals
9. 9. MAT210 2013/14 Sem II 9 of 20 Linear Splines
10. 10. MAT210 2013/14 Sem II 10 of 20 Linear Splines Slope between points
11. 11. MAT210 2013/14 Sem II 11 of 20 v(16) … Again The linear case is unchanged No surprise there
12. 12. MAT210 2013/14 Sem II 12 of 20 Quadratic Splines ● Now things get interesting ● How to find all the coefficients? ● 3n coefficients, n equations, n continuity at end points, whence the other n?
13. 13. MAT210 2013/14 Sem II 13 of 20 2n from continuity Each curve must pass through both endpoints
14. 14. MAT210 2013/14 Sem II 14 of 20 n-1 from smoothness a1 x 2 +b1 x+c1 ⇒2a1 x+b1 a2 x2 +b2 x+c2 ⇒2a2 x+b2 Must match at n-1 interior points 2 a1 xi +b1 =2a2 xi +b2 ∀ i ∈ [1 ,n−1]
15. 15. MAT210 2013/14 Sem II 15 of 20 One more assumption ● This is 3n unknowns and 3n -1 equations ● Need to set one more condition ● Generally set the first spline to be linear ● a1 = 0 ● Now use any technique to solve simultaneous linear equations
16. 16. MAT210 2013/14 Sem II 16 of 20 Revisiting the Rocket
17. 17. MAT210 2013/14 Sem II 17 of 20 The continuous derivatives The draftsman is bending his spline!
18. 18. MAT210 2013/14 Sem II 18 of 20 The Final Matrix
19. 19. MAT210 2013/14 Sem II 19 of 20 The Solution
20. 20. MAT210 2013/14 Sem II 20 of 20 Going Deeper ● The overall curve is smooth and the accuracy can be quite good ● Cubic is better, more common – See that next time ● What about finding the distance traveled? ● From 11 to 14s? ● From 11 to 16s? ● From 0 to 30s?