'If Jed and Chris had met' means that they did not meet.
This sentence is better suited to the 3rd conditional:
'If Chris and Jed had met, they would have discussed mathematics.'
'If Jed and Chris met, they discussed mathematics.' Means we do not know if they met, but if they did, we're sure they talked about mathematics
'When Chris and Jed met, they discussed mathematics'. means they DID meet and they DID discuss mathematics.
For more help on 'if' constructs check this out.
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