## Thursday, December 10, 2015

### HP Prime and Casio Prizm: 3 Point Neville's Method

HP Prime and Casio Prizm: 3 Point Neville's Method

Introduction

Neville’s Method is an interpolation method to find an approximation of f(x) at point x.  The function may or may not be known.  The method uses Lagrange interpolation polynomials.

For this blog entry, we will work with 3 known points to interpolate a fourth point.  We certainly can use a higher order interpolation if desired or needed.  In general for the 3 known points (x0, y0), (x1, y1), and (x2, y2):

 P(0,0) P(0,1) P(1,1) P(0,2) P(1,2) P(2,2)

Think of this like a collapsing bracket, collect two neighboring points until a column is completed.  Repeat the process until P(0,n) is reached.   In this case of 3 points, the approximate answer is y ≈ P(0,2).

Formulae:

P(0,0) = (x0, y0)
P(1,1) = (x1, y1)
P(2,2) = (x2, y2)

P(0,1) = ((x1 – x)*y0) + (x – x0)*y1) / (x1 – x0)
P(1,2) = ((x2 – x)*y1) + (x – x1)*y2) / (x2 – x1)

P(0,2) = ((x2 – x)*P(0,1) + (x – x0)*P(1,2)) / (x2 – x0)
Y = P(0,2)

The NEVILLE3 program works with the 3-point interpolation where lists presents the points {x0, x1, x2} and {y0, y1, y2}.  If you use a TI-84 Plus, the Casio Prizm version can be adopted (with appropriate changes).

HP Prime Program:  NEVILLE3

Input:  lx and ly and list of three elements, {x0, x1, x2} and {y0, y1, y2}, respectively, and x is the target point.

EXPORT NEVILLE3(lx,ly,x)
BEGIN
// Neville 3-Point Approx.
// EWS 2015-12-07

LOCAL la,a,y,k,n;

la:={0}; // for consistency

FOR k FROM 1 TO 3 DO

IF k==3 THEN
a:=((lx(3)-x)*la(2)+(x-lx(1))*
la(3))/(lx(3)-lx(1));
ELSE

a:=((lx(k+1)-x)*ly(k)+(x-lx(k))*
ly(k+1))/(lx(k+1)-lx(k));
END;
la:=CONCAT(la,{a});
END;
y:=la(4); RETURN y;
END;

Casio Prizm Program NEVILLE3

Input:  List 1 and List 2 and list of three elements, {x0, x1, x2} and {y0, y1, y2}, respectively, and x is the target point.

“3 PT NEVILLE”
“LIST X”? → List 1
“LIST Y”? → List 2
“X?” → X
{0} → List 3
For 1 → K To 3
If K = 3
Then
(( List 1 – X ) * List 3 + ( X – List 1) * List 3 )
÷ (List 1 – List 1) → A
Else
(( List 1[K+1] – X ) * List 2[K] + (X – List 1[K]) * List 2[K+1] )
÷ (List 1[K+1] – List 1[K] ) → A
IfEnd
Augment(List 3,{A}) → List 3
Next
List 3 → Y
Y

Examples:

Example 1;
List X/List 1 = {16, 64, 100}
List Y/List 2 = {0.25, 0.125, 0.1}
X = 81

Result:  0.1058511078

Example 2:
List X/List 1 = {-2, 0, 2}
List Y/List 2 = {11, 4, 11}
X = 3

Result:  19.75

Sources:

Mitchell, Kevin.  “Neville’s Method”  Simon Fraser University. MACM 316 – Fall 2010.   http://people.math.sfu.ca/~kevmitch/teaching/316-10.09/neville.pdf    Retrieved December 4, 2015

Wikiversity.  “Topic:  Numerical analysis/Neville’s algorithm examples”  https://en.wikiversity.org/wiki/Topic:Numerical_analysis/Neville%27s_algorithm_examples  Retrieved December 4, 2015

This blog is property of Edward Shore.  2015