Chapter 05.03 Newton’s Divided Difference Interpolation

1 Chapter newton s Divided Difference Interpolation After reading this Chapter , you should be able to: 1. derive newton s Divided Difference method of Interpolation , 2. apply newton s Divided Difference method of Interpolation , and 3. apply newton s Divided Difference method interpolants to find derivatives and integrals. What is Interpolation ? Many times, data is given only at discrete points such as ,,00yx 11,yx, .., 11, nnyx, nnyx,. So, how then does one find the value of y at any other value of x? Well, a continuous function xf may be used to represent the 1 n data values with xf passing through the 1 n points (Figure 1). Then one can find the value of y at any other value of x. This is called Interpolation . Of course, if x falls outside the range of x for which the data is given, it is no longer Interpolation but instead is called extrapolation. So what kind of function xf should one choose?

2 A polynomial is a common choice for an interpolating function because polynomials are easy to (A) evaluate, (B) differentiate, and (C) integrate, relative to other choices such as a trigonometric and exponential series. Polynomial Interpolation involves finding a polynomial of order n that passes through the 1 n points. One of the methods of Interpolation is called newton s Divided Difference polynomial method. Other methods include the direct method and the Lagrangian Interpolation method. We will discuss newton s Divided Difference polynomial method in this Chapter . newton s Divided Difference Polynomial Method To illustrate this method, linear and quadratic Interpolation is presented first. Then, the general form of newton s Divided Difference polynomial method is presented. To illustrate the general form, cubic Interpolation is shown in Figure 1.

3 Chapter Figure 1 Interpolation of discrete data. Linear Interpolation Given ),(00yx and ),,(11yx fit a linear interpolant through the data. Noting )(xfy and )(11xfy , assume the linear interpolant )(1xf is given by (Figure 2) )()(0101xxbbxf Since at 0xx , 00010001)()()(bxxbbxfxf and at 1xx , )()()(0110111xxbbxfxf )()(0110xxbxf giving 01011)()(xxxfxfb So )(00xfb 01011)()(xxxfxfb giving the linear interpolant as )()(0101xxbbxf )()()()()(0010101xxxxxfxfxfxf 00,yx 11,yx 22,yx 33,yx xfxy newton s Divided Difference Interpolation Figure 2 Linear Interpolation . Example 1 The upward velocity of a rocket is given as a function of time in Table 1 (Figure 3). Table 1 Velocity as a function of time. )s( t)m/s( )(tv0 0 10 15 20 30 Determine the value of the velocity at 16 t seconds using first order polynomial Interpolation by newton s Divided Difference polynomial method.

4 Solution For linear Interpolation , the velocity is given by )()(010ttbbtv Since we want to find the velocity at 16 t, and we are using a first order polynomial, we need to choose the two data points that are closest to 16 t that also bracket 16 t to evaluate it. The two points are 15 t and 20 t. Then ,150 )(0 tv ,201 )(1 tv gives )(00tvb 00,yx 11,yx xf1xy Chapter 01011)()(tttvtvb Figure 3 Graph of velocity vs. time data for the rocket example. Hence )()(010ttbbtv ),15( t 2015 t At ,16 t )1516( )16( v m/s If we expand ),15( )( ttv 2015 t we get , )(ttv 2015 t and this is the same expression as obtained in the direct method. Quadratic Interpolation Given ),,(00yx ),,(11yx and ),,(22yx fit a quadratic interpolant through the data.

5 Noting ),(xfy ),(00xfy ),(11xfy and ),(22xfy assume the quadratic interpolant )(2xf is given by ))(()()(1020102xxxxbxxbbxf newton s Divided Difference Interpolation At 0xx , ))(()()()(100020010002xxxxbxxbbxfxf 0b )(00xfb At 1xx ))(()()()(110120110112xxxxbxxbbxfxf )()()(01101xxbxfxf giving 01011)()(xxxfxfb At 2xx ))(()()()(120220210222xxxxbxxbbxfxf ))(()()()()()(1202202010102xxxxbxxxxxfxf xfxf Giving 02010112122)()()()(xxxxxfxfxxxfxfb Hence the quadratic interpolant is given by ))(()()(1020102xxxxbxxbbxf ))(()()()()()()()()(100201011212001010xx xxxxxxxfxfxxxfxfxxxxxfxfxf Figure 4 Quadratic Interpolation . 00,yx 11,yx 22,yx xf2y Chapter Example 2 The upward velocity of a rocket is given as a function of time in Table 2.

6 Table 2 Velocity as a function of time. )s( t(m/s) )(tv0 0 10 15 20 30 Determine the value of the velocity at 16 t seconds using second order polynomial Interpolation using newton s Divided Difference polynomial method. Solution For quadratic Interpolation , the velocity is given by ))(()()(102010ttttbttbbtv Since we want to find the velocity at ,16 t and we are using a second order polynomial, we need to choose the three data points that are closest to 16 t that also bracket 16 t to evaluate it. The three points are ,100 t ,151 t and 202 t. Then ,100 )(0 tv ,151 t )(1 tv ,202 )(2 tv gives )(00tvb 01011)()(tttvtvb 02010112122)()()()(tttttvtvtttvtvb newton s Divided Difference Interpolation Hence ))(()()(102010ttttbttbbtv ),15)(10( )10( ttt 2010 t At ,16 t )1516)(1016( )1016( )16( v m/s If we expand ),15)(10( )10( )( ttttv 2010 t we get )(tttv , 2010 t This is the same expression obtained by the direct method.

7 General Form of newton s Divided Difference Polynomial In the two previous cases, we found linear and quadratic interpolants for newton s Divided Difference method. Let us revisit the quadratic polynomial interpolant formula ))(()()(1020102xxxxbxxbbxf where )(00xfb 01011)()(xxxfxfb 02010112122)()()()(xxxxxfxfxxxfxfb Note that ,0b,1b and 2b are finite Divided differences . ,0b,1band 2b are the first, second, and third finite Divided differences , respectively. We denote the first Divided Difference by )(][00xfxf the second Divided Difference by 010101)()(],[xxxfxfxxf and the third Divided Difference by 020112012],[],[],,[xxxxfxxfxxxf 0201011212)()()()(xxxxxfxfxxxfxf where ],[0xf],,[01xxf and ],,[012xxxf are called bracketed functions of their variables enclosed in square brackets. Rewriting, ))(](,,[)](,[][)(1001200102xxxxxxxfxxxxf xfxf Chapter This leads us to writing the general form of the newton s Divided Difference polynomial for 1 n data points, nnnnyxyxyxyx.

8 ,,,,111100 , as ))..()((..)()(110010 nnnxxxxxxbxxbbxf where ][00xfb ],[011xxfb ],,[0122xxxfb ],..,,[0211xxxfbnnn ],..,,[01xxxfbnnn where the definition of the thm Divided Difference is ],..,[0xxfbmm 0011],..,[],..,[xxxxfxxfmmm From the above definition, it can be seen that the Divided differences are calculated recursively. For an example of a third order polynomial, given ),,(00yx ),,(11yx ),,(22yx and ),,(33yx ))()(](,,,[))(](,,[)](,[][)(210012310012 00103xxxxxxxxxxfxxxxxxxfxxxxfxfxf Figure 5 Table of Divided differences for a cubic polynomial. Example 3 The upward velocity of a rocket is given as a function of time in Table 3. 00xfx0b 11xfx 22xfx 33xfx 1b2b3b 01,xxf 12,xxf 23,xxf 012,,xxxf 123,,xxxf 0123,,,xxxxfNewton s Divided Difference Interpolation Table 3 Velocity as a function of time.

9 (s) t(m/s) )(tv0 0 10 15 20 30 a) Determine the value of the velocity at 16 t seconds with third order polynomial Interpolation using newton s Divided Difference polynomial method. b) Using the third order polynomial interpolant for velocity, find the distance covered by the rocket from s 11 t to s 16 t. c) Using the third order polynomial interpolant for velocity, find the acceleration of the rocket at s 16 t. Solution a) For a third order polynomial, the velocity is given by ))()(())(()()(2103102010ttttttbttttbttbb tv Since we want to find the velocity at ,16 tand we are using a third order polynomial, we need to choose the four data points that are closest to 16 t that also bracket 16 t to evaluate it. The four data points are ,100 t ,151 t ,202 t and t.

10 Then ,100 t )(0 tv ,151 t )(1 tv ,202 t )(2 tv , )(3 tv gives ][00tvb )(0tv ],[011ttvb 0101)()(tttvtv ],,[0122tttvb 020112],[],[ttttvttv Chapter 121212)()(],[tttvtvttv ],[01 ttv 0201122],[],[ttttvttvb ],,,[01233ttttvb 03012123],,[],,[tttttvtttv 131223123],[],[],,[ttttvttvtttv 232323)()(],[tttvtvttv 121212)()(],[tttvtvttv 131223123],[],[],,[ttttvttvtttv ],,[012 tttv 030121233],,[],,[tttttvtttvb Hence ))()(())(()()(2103102010ttttttbttttbttbb tv newton s Divided Difference Interpolation )20)(15)(10( )15)(10( )10( tttttt At ,16 t )2016)(1516)(1016( )1516)(1016( )1016( )16(3 v m/s b) The distance covered by the rocket between s 11 t and s 16 t can be calculated from the interpolating polynomial )20)(15)(10( )15)(10( )10( )(3 tttttttv , t Note that the polynomial is valid between 10 t and t and hence includes the limits of 11 t and 16 t.