A linear formulation with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif"><mml:mrow><mml:mi>O</mml:mi><mml:mo stretchy="false">(</mml:mo><mml:msup><mml:mi>n</mml:mi><mml:mn>2</mml:mn></mml:msup><mml:mo stretchy="false">)</mml:mo></mml:mrow></mml:math> variables for quadratic assignment problems with Manhattan distance matrices