Wikipedia: Algebraic derivation of mean says:

Sure, is "a discrete random variable", but does that inevitably mean that ? Couldn't all be, say, multiples of 3 ( )?

And in next step they use this to saywhich is an essential part of proving that if (which is why I'd like to understand why should ).Quote:

The first term of the series (with indexk= 0) has value 0 since the first factor,k, is zero.