HP 12c Solutions Handbook page 84

A useful curve for evaluating sales trends, etc., is the Gompertz curve.
This is a "growth" curve having a general "S" shape and may be used to
describe series of data where the early rate of growth is small, then
accelerates for a period of time and then slows again as the time grows
long. The sales curve for many products follow this trend during the
introductory, growth and maturity phases.
The data points to be fit to a Gompertz curve should be equally spaced
along the x (or time) axis and all the data points must be positive. The
points are divided serially into 3 groups for data entry.
The following HP 12C program processes the data, fits it to a Gompertz
curve and calculates estimated values for future data points. The 3
constants which characterize the curve are available to the user if desired.
KEYSTROKES
CLEAR
1
4
3
2
2
1
3
2
1
4
00
DISPLAY
00-
01- 43 23
02-44 40
03- 33
04- 43 23
05-44 40
06- 33
07- 43 23
08-44 40
09-
10-44 40
11- 45
12-43, 33 00
13- 45
14- 45
15- 30
16- 45
17- 45
83
3
2
1
1
4
4
3
2
2
1