Sum interpretation Is the interpretation of $$ \sum_{1 \leq j-1 \leq n}a_{j-1} $$ in that stile $$ \sum_{j-1=n}^{n}a_{j-1} $$ correct ? Can somebody give me a numerical example of that sum, please ?

You can write it as

$$\sum_{j = 1}^{n} a_j$$