Consider the following scenario.
I am researching on a problem in a domain and I don't know anything about the existence of Pythagoras theorem.
After so much time of my research I figured myself that if a fact like:
the square of a side which is opposite to the 90 degrees in a triangle is equal to the sum of squares of remaining sides
exists, then my research completes and I can publish a good paper. But I never encountered Pythagoras theorem during my search of literature.
If I came to know about its existence during a discussion with a colleague or any random person, then is there any ethical need to keep him as a co-author since I struggled a lot of time and didn't find by myself?