What comes next?

[Thomas Klemm]

(10-13-2024 08:49 PM)John Keith Wrote:  Of course now you need the ISPRIME? function, which limits you to the HP 49 or later unless you want to roll your own ISPRIME?.

In case of Mersenne prime numbers we can use the Lucas–Lehmer primality test.

Here's an implementation for the HP-42S:

Code:

00 { 32-Byte Prgm }
01▸LBL "LLT"
02 RCL ST X
03 2
04 STO- ST Z
05 X<>Y
06 Y↑X
07 1
08 -
09 4
10▸LBL 00
11 X↑2
12 2
13 -
14 RCL ST Y
15 MOD
16 DSE ST Z
17 GTO 00
18 END

Examples

3 XEQ "LLT"
0

5 XEQ "LLT"
0

7 XEQ "LLT"
0

11 XEQ "LLT"
1'736

13 XEQ "LLT"
0

17 XEQ "LLT"
0

19 XEQ "LLT"
0

23 XEQ "LLT"
6'107'895

29 XEQ "LLT"
458'738'443

31 XEQ "LLT"
0