Ah, the elusive HP-10C. The smallest member of Hewlett-Packard's Voyager family of calculators, it is surprisingly difficult to find these days. I am delighted to have gotten my hands on one that, even though it's in less than perfect cosmetic condition, still operates flawlessly.

The HP-10C has a much more limited amount of memory than its bigger cousins. At startup, you have 9 program steps and 10 memory registers available; the maximum amount of program memory is 79 program steps, with no storage registers left.

In such a limited amount of memory, it's quite difficult to implement programs such as a polynomial approximation for the Gamma function. What I have here instead is another method of approximation, using the incomplete Gamma function. With an integration limit of 24, the accuracy of the result is usually 10 digits. The program fits neatly into the calculator, with the only down side being its slow speed of execution; obtaining a result may take up to two minutes.

01	44  1	STO 1
02	 1	1
03	44  0	STO 0
04	35	CLX
05	 0	0
06	42 10	x<=y
07	22 14	GTO 14
08	45  1	RCL 1
09	44 10 0	STO÷ 0
10	 1	1
11	40	+
12	44  1	STO 1
13	22 05	GTO 05
14	 1	1
15	45  1	RCL 1
16	42 10	x<=y
17	22 23	GTO 23
18	 1	1
19	30	-
20	44 20 0	STOX 0
21	44 01	STO 1
22	22 14	GTO 14
23	 2	2
24	 4	4
25	34	x-y
26	14	yx
27	45  1	RCL 1
28	10	÷
29	44  2	STO 2
30	 2	2
31	 4	4
32	45  1	RCL 1
33	 1	1
34	40	+
35	44  1	STO 1
36	10	/
37	45  2	RCL 2
38	20	×
39	44  2	STO 2
40	40	+
41	42 10	x<=y
42	22 44	GTO 44
43	22 30	GTO 30
44	 2	2
45	 4	4
46	12	ex
47	10	÷
48	45  0	RCL 0
49	20	×