Hewlett-Packard [HPQ  Loading...      ()   ] shares gained more than two percent in today's session, after its board approved an additional $10 billion stock buyback.

In the past three months, 41 companies in the S&P 500 have made such announcements.  Wal-Mart [WMT  Loading...      ()   ] is at the top of the list, with a transaction value of $15 billion. 

When cash-rich companies decide to repurchase their own shares, investors tend to see the move as a sign of confidence. The underlying message is that a firm's stock may be "undervalued," prompting management to invest in their own stock.

• The Biggest Stock Buybacks of the Decade

Repurchase programs could also be seen as an effort to change the supply of a company's stock, therefore affecting other valuation metrics. Note that while corporations may approve a certain figure over a specific period  of time, the company is not required to buy back the entire amount. 

Here is a look at the 15 largest buyback announcements in the last three months:

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