Alaska Air Group said its AAG Holdings subsidiary has extended its offer to purchase all the outstanding shares of Long Beach-based Jet America for $4.50 a share until Oct. 2 . . . Kaufman & Broad said it expects to report third-quarter earnings per share of between 20 cents and 22 cents, up from 10 cents per share a year earlier . . . New York-based Infinity Broadcasting has completed the purchase of KROQ-FM, Los Angeles, from Mandeville Broadcasting for about $41 million . . . Texas Air Corp. said it is seeking speedy approval for its proposed purchase of People Express partly because of People's poor financial condition.