The formula on the board should really read d u r . p a l = p a l ⋅ d u p a l {\displaystyle dur.pal=pal\cdot dupal} , where d u r . A ( u 1 , … , u r ) = ( u 1 + ⋯ + u r ) A ( u 1 , … , u r ) {\displaystyle dur.A(u_{1},\ldots ,u_{r})=(u_{1}+\cdots +u_{r})A(u_{1},\ldots ,u_{r})} .
, where 
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