The parallelogram identity on groups and deformations of the trivial character in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi mathvariant="normal">SL</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:mi>ℂ</mml:mi><mml:mo>)</mml:mo></mml:mrow></mml:mrow></mml:math>
Journal de l'École polytechnique. Mathématiques/Journal de l'École polytechnique. Mathématiques