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I want to write the following expression in Einstein notation:

[tex](a_1+\ldots+a_n)(b_1+\ldots+b_n)[/tex]

I tried it by introducing the term [tex]I^{ij}=1[/tex] and writing:

[tex]a_ib_jI^{ij}[/tex]

Are you aware of another, more convenient form for this?

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# Einstein notation question

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