A diagram for the left trefoil can be obtained by reversingall the crossings in a diagram for the right trefoil:make every overcrossing into an undercrossing.
If we rewrite the skein relation
t-1[t] - t [t] =(t1/2 - t-1/2)[t]
so as to interchange the positionof the undercrossing and the overcrossing, we obtain
- t [t]+t-1[t]=(t1/2 - t-1/2)[t].
Now let's multiply both sides of the relation by -1:
t [t] -t-1[t]=(t-1/2 - t1/2)[t].
This manipulation shows that the skein relation holds if the undercrossingand the overcrossing are interchanged and at the same time positiveand negative powers of t are interchanged.
If we apply this form of the skein relation to the left trefoil, thecalculation will proceed exactly as it did for the right trefoil,except that the exponents of t will be exactly oppositefrom what they were before. At the end we will obtain
[t] = - t-4 + t-3 +t-1
which is different from the value for the right trefoil!