The investors who sell are wealthier by amount $X because now they have fewer shares and more dollars.
The investors who don't sell are wealthier by the same amount $X because the shares they kept are worth more, because prices go up.
> keeping the share price initially constant. This statement is definitely incorrect, unless you're being very technicaly and pedantic about "initially". You can think about it theoretically or you can look at empirical evidence. It is well-supported empirically that share prices go up after buybacks, and in fact they do so quantitatively by exactly the amount necessary for the equation implied above to hold.
