Accurate Normal Distribution for the HP67/97

[Albert Chan]

Hi, John Keith

Regarding how to split z for 1 Exp Method Revision 1, note the reason for the split:

Z(z) = Z(x + h) = Z(x) exp(-x h - h²/2) = Z(x) exp(y)

The split have to ensure *BOTH* good Z(x) and exp(y).
If |z| is limited to below √2000 ~ 44.7, FIX-4 work well.

Above this z range, x = -5 RND of z, (to evaluate Z(x) correctly).
|y| is now too big, exp(y) need more terms to get "good enough".

Instead of updating revision 1 with above, I choose an easier way.
1 Exp Method Revision 2 only need a linear correction, and work for even bigger |z|

Example: Z(99.1234567), assuming calculator can handle bigger exponent

B = z²/2 = 4912.72 983408
D = exp(-B) / √(2 Pi) = 1.07016 820397 e-2134
x = z to 5 digits = 99.123
h = z - x = 0.0004567
y = B - x²/2 - x h - h²/2 = 161.2555 e-11

Z(z) = D + y D = D + 173 ULP = 1.07016 820570 e-2134, error = +1 ULP