HP-71B Program to Find The Root Of A Function

by Namir Shammas

The following program calculates the root of a function using Richmond's algorithm. This algorithm uses the following equation to update the guess for the root:

x' = x – 2 f(x)f'(x)/{2 [f’(x)]² – f(x) f''(x)}

where f(x) is the function whose root is sought, f '(x) is the first derivative of function f(x), and f ''(x) is the second derivative of function f(x). The program approximate the derivatives using the following difference approximations:

f '(x) ≈ (f(x + h) - f(x-h)) / 2h

f''(x) ≈ (f(x+h) - 2 f(x) + f(x-h))/ h²

where h = 0.01 * (1 + |x|)

The pseudo-code for the Richmond algorithm is:

Obtain initial guess x and Tolerance

Obtain or set value to MaxIterations

Clear Diverge flag

Set IterCount = 0

Loop

Increment IterCount

If IterCount > MaxIterations Then Set Diverge flag and exit loop

h = 0.01 * [1 + |x|]

Let f0 = f(x), fp = f(x+h), and fm = f(x-h)

Let D1 = (fp – fm) / (2h), and D2 = (fp – 2f0 + fm) / (h*h)

diff = 2 f0 * D1/(2*D1^2 – f0*D2)

x = x – diff

Until |diff| < Tolerance

Return root as x and Diverge flag

The program prompts you to enter:

1. Guess for the root.

2. Tolerance for the root. The default value is 1E-7.

3. The maximum number of iterations. The default value is 55.

The program displays the following results:

1. The root value.

2. The number of iterations.

If the number of iterations exceeds the maximum limit, the program displays the text SOLUTION FAILED before displaying the above results.

Here is a sample session to find the root of f(x) = e^x - 3*x^2 near x = 5 and using the default values for the tolerance and maximum number of iterations:

PROMPT/DISPLAY	ENTER/PRESS
>	[RUN]
GUESS?	5[END LINE]
TOLER? 1E-7	[END LINE]
MAX ITERS? 55	[END LINE]
(Audio beep)
ROOT= 3.73307902863	[CONT]
ITERS= 5

Here is the BASIC listing:

10 DEF FNF(X) = EXP(X) - 3 * X^2
20 INPUT "GUESS? ";X
30 INPUT "TOLER? ","1E-7";A$
40 T = VAL(A$)
50 INPUT "MAX ITER? ","55";A$
60 M = VAL(A$)
70 I = 0
80 REM START
90 I = I + 1
100 IF I > M THEN 200
110 H = 0.01 * (1 + ABS(X))
120 F0 = FNF(X)
130 F1 = FNF(X+H)
140 F2 = FNF(X-H)
150 D1 = (F1 - F2) / 2 / H
160 D2 = (F1 - 2 * F0 + F2) / H^2
170 D = 2 * F0 * D1/(2 * D1^2 - F0 * D2)
180 X = X - D
190 IF ABS(D) > T THEN 80
200 REM STOP
210 BEEP
220 If I > M THEN DISP "SOLUTION FAILED" @ PAUSE
230 DISP "ROOT = ";X
240 PAUSE
250 DISP "ITERS = ";I
260 END

The program uses the variables shown in the following table:

Variable Name	Contents
X	Guess for root
T	Tolerance
M	Maximum number of iterations
I	Iteration counter
H	Increment h
F0	Value of the function at X
F1	Value of the function at X+h
F2	Value of the function at X-h
D1	Value of the first derivative f'(X)
D2	Value of the second derivative f''(X)
D	Root refinement
A$	Temporary input for T and M

You can customize the above listing by changing the definition of function FNF in line 10. The current listing is set to solve the following equation:

f(x) = e^x - 3 * x^ 2

The above equation has roots near -0.45, 0.91, and 3.73.

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