Howard Hughes Holdings surges on proposal to merge with unit of Ackman's Pershing Square

** Shares of real estate developer Howard Hughes Holdings HHH up 9.1% at $78.29 in morning trading

** Billionaire investor Bill Ackman's Pershing Square PSH , which along with its affiliates, has a 37.6% stake in the company, proposes merger between Howard Hughes and a Pershing unit

** Ackman also says Pershing Square is increasing its stake in Howard Hughes Holdings by $1 billion

** Texas-based HHH has market capitalization of $3.6 billion, according to data compiled by LSEG

** Stock hit a four-month low on Friday, down about 18% from its 2-1/2 year high in November

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