NEW YORK -- Just six months after Carl Icahn shed the last of his holdings in Mentor Graphics, another billionaire activist investor is going after the firm.

Hedge fund Elliott Management yesterday disclosed it has accumulated an 8.1% stake in Mentor, and immediately used the moment to claim the shares are undervalued.

According to the hedge fund, Mentor's stock should be about 20% higher than they were a day ago.

Mentor experienced a long and public proxy fight with Icahn starting in 2010 after the famed investor starting buying up shares in the company. Icahn eventually accumulated a 14% ownership stake and managed to place three directors on the software firm's board. The last of those directors left in February 2015, and Mentor bought the remainder of Icahn's holdings in May of this year.

Elliott is co-owned by Paul Singer, whose net worth was estimated by Forbes as in excess of $2 billion.