Why is it necessary to change the value in the "Begin with" field when accessing the previous elements? The sequence of values has a beginning or a boundary. There is nothing beyond the beginning. All calculations are based on the current value of i, so when we start counting the value for i=0, i.e. for the first value of the sequence, and try to refer, for example, to the previous value, the program will say that it is empty, and it will not be able to calculate. However, when the entry is basic, for example [i-9], it is not necessary to set the "Begin with" – the program will detect it automatically.