This question arose after I saw a youtube-vid where Grandi's series was discussed.
It seems that the sum of the series will be 0 for an even, and 1 for an odd number of terms, where a term is defined as (-1)n, n indicating the n'th term.
It seems that even when s is derived that the above tacit assumptions of even/odd is used.
Is it valid to make these assumptions?
(My lay-opinion is that these assumptions are wrong, and therefor also the derived value of s (= 1/2), and that the series has no sum as stated in the wikipedia entry)
You are exactly right. An "infinite number of terms" is neither even nor odd. With Grandi's series, you can only discuss the values of partial sums of the series - because the series itself does not converge, and therefore does not have any value.
A series is defined as the limit of its sequence of partial sums, if that limit exists: $$ \sum_{n=0}^{\infty}a_n=\lim_{N\rightarrow\infty}\sum_{n=0}^{N}a_n $$ For Grandi's series, the sequence of partial sums alternates between 0 and 1, and does not converge; hence the series itself is undefined.