Here we sum over all decompositions <math>c_d \otimes \ldots \otimes c_1 = (c''= c_d \otimes \ldots \otimes c_{m+n+1}) \otimes (c'= c_{m+n} \otimes \ldots \otimes c_{n+1}) \otimes (c= c_n \otimes \ldots \otimes c_1 ) </math> into pure tensors of lengths <math>d-m-n,m,n\geq 0</math>, in particular allow <math>c'=1 \;\; {\scriptstyle (\text{or}\; c=1 \;\text{or}\; c''=1 )} </math> to have length <math>|c'|=0</math>. | Here we sum over all decompositions <math>c_d \otimes \ldots \otimes c_1 = (c''= c_d \otimes \ldots \otimes c_{m+n+1}) \otimes (c'= c_{m+n} \otimes \ldots \otimes c_{n+1}) \otimes (c= c_n \otimes \ldots \otimes c_1 ) </math> into pure tensors of lengths <math>d-m-n,m,n\geq 0</math>, in particular allow <math>c'=1 \;\; {\scriptstyle (\text{or}\; c=1 \;\text{or}\; c''=1 )} </math> to have length <math>|c'|=0</math>. |