Paper-based algorithm to find longest formula which is common to at least two formulas Given a list of logical formulas:


f1: A&B
f2: A&D&C
f3: B&D&E
f4: A&B&C
f5: 
fn: ... 


In this case I want A&amp;B as the longest formula which is common to at least two formulas.
Is there a simple algorithm to find the longest formula that could be performed on paper?
For each pair of formulas $f_i,f_j$ from your list, find the longest formula common to both $f_i,f_j$.  If things are as in your example, this is as simple as a set intersection.  That gives you $n(n-1)/2$ candidates.  Take the candidate that is the longest.