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This no question to the Problem 955 (View Problem)
In the first approach I read the problem and asked myself: „What the hell is a triangle number?“. A few lines later, there was a hint for the red marked numbers in the example.
I tried to find a algo for this.
At this state I copied the problem description (html-sign in right corner of problem box) to my markup box in Jupyter and wondered why I get a different text in comparison to the html-view in Safari (MAC).
So I discovered that there is a popup box describing the calculation for a triangle number in the web view.

For me this was a new behavior in describing problems.

Wouldn’t it be better if this triangle description was visible directly in the problem description?
Yesterday evening this popup was not popping up (I don’t know why). Today it will. Strange.

The thin underline is a sign that there is a popup box for it.
When you hover your mouse over it the the popup will show.
This is a feature of the site that has occured many times in previous problems.
Moreover if you google on 'triangle number' you will find the necessary information too.

Hello hk
For my amount of solved problems I never saw an underlined text.
My focus was on the bolded text and I didn't saw the underline.

I haven't tried to search for 'triangle number', because I used the info from the popup in the second step.
I'm close to the solution with index 54 but I think I have to rebuild my program, because surly I have missed an idea to speedup the solution finding.
This thinking process is, what I love to be part of Project Euler