Optimal spectral approximation of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="mml52" display="inline" overflow="scroll" altimg="si52.gif"><mml:mn>2</mml:mn><mml:mi>n</mml:mi></mml:math>-order differential operators by mixed isogeometric analysis
Computer methods in applied mechanics and engineering