Determine if a relation is transitive
Haskell, 42 bytes

f x=and[elem(a,d)x|(a,b)&lt;-x,(c,d)&lt;-x,b==c]


Usage example: f [(1,2), (2,4), (6,5), (1,4)]-> True.

(Outer)loop over all pairs (a,b) and (inner)loop over the same pairs, now called (c,d) and every time when b==c check if (a,d)is also an existent pair. Combine the results with logical and.