Project Euler Forum

I haven't done this problem yet, and have no idea how, but 10^9 should not be difficult to brute force, so I don't see a real need for such a hint.

Yes this one seems to be a hard one but there are 10 to 20 problems I spent more time to solve than this one.

I had found an effective algo on the first day. I even tried to do it by hand (later I checked that I had found 50% of the numbers by hand) then I switched to coding which took most of my time until I solved it.

Just wondering, how many k+1/2 numbers are below the 10^18 limit? I've been able to find 15...

Too few.

I know that's too few, that's why i'm asking how many there are hehehe

Okay, so then: in absolute numbers, you've not missed many, the percentage of those you missed is significant, though.

I have so far found 21 such numbers < 10^18 using manual search (of course guided by a selection algorithm using Sage). Also my manual selection algorithm stumbled on few that exceeded 10^18. I believe I know the algorithm but I suspect it is a bit awkward to march through the multipliers. Now I am writing the algorithm to make sure if I have missed some. I suspect I am not that far off from the actual numbers that are less than 10^18 and produce k + 1/2

In my opinion this problem is not as hard as many people think (judging by the number of solvers..)