Prime number finder HP11C?

[jbhp55]

Dear Don,

'the whole thing' is in fact quite short:

0xP+L(K:1)x0+PRIME=(L(C:1)-C+SIGMA(D:3:SQRT(P)+1:2:IF(FP(P/D)<>0:1+L(C)x0+L(K:0)x0)))xK+PxK+L(P+2)x0

(SIGMA means the Greek capital, of course)

Use: type starting number (must be odd), press P
outcome is 0 if P is not a prime, otherwise P is given. Press PRIM to continue search. (new P is two higher than original)

This equation has evolved and is not optimized for shortness. Also, I don't remember why C and K are output. The advanced guide for the 19B and 28C/S gives the way to suppress them (quite easy). The main algorithm is standard for the solver. It seems quite rough, as the search for a divisor is always continued up to the square root, but that is the consequence of the fact that you cannot branch out of a sum. Nevertheless, the search is quite quick, as I mentioned earlier.

best regards,

Jaap.