Thats interesting. Obviously the behaviour of how expressions with range variables are evaluated and assigned has changed.Or is it the if-functions which acts differently now? Don't know, would have to further investigate.

But easy solutions are at hand quickly. Most obvious one is to define the first value in P in a separate expression.

It does not matter what value P[0 is assigned to - Prime just insists that P[0 to be defined. So I guess the problem has to do with the combination of shortcut evaluation of if and usage of ranges - not sure, though.

The if-function is obsolete now and you could also use

In your case you might also consider replacing the recursion by a more direct calculation

S

StuartBruff

23-Emerald V

It would be intersesting to look at the help for "if" in Prime (I don't have it and can't download to my work PC).

In M15, if(cond,x,y) only evaluates y if cond is false, hence for n=0, M15 doesn't evaluate P_nn-1 = P_0-1 and therefore doesn't complain about the invalid index.

Stuart