The Los Angeles Business Journal’s index of largest local public companies gained strongly for the week as two companies announced they were being acquired.

The weighted LABJ Stock Index rose 3.6 percent to 160.88 for the trading week ending April 26. Gainers outpaced decliners 126 to 32, with 13 companies unchanged.

Merger announcements fueled two of the biggest gainers. Power-One Inc. soared 57 percent to $6.21 after Swiss industrial giant ABB Ltd. signed a deal to buy the Camarillo power-conversion technology company for about $1 billion.

MPG Office Trust Inc. jumped 25 percent to $3.12. New York’s Brookfield Office Properties Inc. announced that a subsidiary would acquire the downtown L.A. commercial office building owner in a deal valued at $180 million.

First-quarter financial reports also were a major driver. Stamps.com Inc., an El Segundo provider of Internet-based postage services, gained 40 percent to $33.14, after reporting a better-than-expected quarter. Calabasas homebuilder Ryland Group Inc. rose 26 percent to $45.25, also on a good earnings.

Among the biggest decliners was Virco Mfg. Corp., falling 19 percent to $2.02. The Torrance maker of institutional furniture reported a larger quarterly loss and warned of continuing volatility and weaker demand in its core K-12 public school market.