(HP65) Factorial and Gamma Function

[Gamo]

Just noticed that HP 65 cannot calculate decimal Factorial and HP67 app for Android also cannot do it.

Here is a handy program using Stirling's approximation for Factorial and Gamma Function.

This approximation is good for x<70

Program:

Code:


LBL A
571
ENTER
2488320
/
CHS
STO 1
139
ENTER
51840
/
CHS
STO 2
288
1/x
STO 3
12
1/x
STO 4
CLx
RTN
LBL E
ENTER
STO 5
2
x
pi
x
Sqr
RCL 5
RCL 5
2
/
Y^x
STO 6
x
RCL 5
e^x
1/x
x
STO x6
RCL 5
1/x
ENTER
ENTER
ENTER
RCL 1
x
RCL 2
+
x
RCL 3
+
x
RCL 4
+
x
1
+
RCL 6
x
RTN

Instruction:
1. Press [A] Initialize
2. Press [E] for x!

Example: [A] Initialize then 4.25 [B]
result approximation 35.21