So for example, in the Diels-Alder reaction (which is
suprafacial with respect to both ethene and cyclobutadiene):
qs = 4
(butadiene)
ps =2 (ethene)
p + q = 6 (total number of p electrons)
6 can be written as 4n+2,
(with n=1), so the thermal
Diels-Alder reaction is allowed.
Similar generalizations have been found which give rules
for all pericyclic reactions