On <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si1.gif" overflow="scroll"><mml:mo stretchy="false">(</mml:mo><mml:mi>N</mml:mi><mml:mo>,</mml:mo><mml:mi>ϵ</mml:mi><mml:mo stretchy="false">)</mml:mo></mml:math>-pseudospectra of operators on Banach spaces