1,3,6,10,15,21,28.

Somebody tells you a random positive integer of any size. Is there a quick way to work out if it is a triangle number or not (in other words, without going through all of the triangle numbers untill you get to a number as high or higher as the integer you've been told)?

In the last line of your post you say "I'm sure other, cleaner methods will be found"

I am not so sure. This is the same method that immediately occured to me and really, it is about as simple as one could expect.

But I confess I did have to look up "Triangle number" just to be sure it meant what I inferred from Lewis' brief description.

Posted by Brian Wainscott on 2003-08-07 16:48:37