We introduce the α,β-symmetric difference derivative and the
α,β-symmetric N\"orlund sum. The associated symmetric quantum
calculus is developed, which can be seen as a generalization of the forward and
backward h-calculus.Comment: Submitted 26/Sept/2011; accepted in revised form 28/Dec/2011; to
Proceedings of International Conference on Differential & Difference
Equations and Applications, in honour of Professor Ravi P. Agarwal, to be
published by Springer in the series Proceedings in Mathematics (PROM