q-Analogs of classical 6-periodicity: from Euler to Chebyshev
  The sequence of period 6 starting with 1, 1, 0, -1, -1, 0 appears in many different disguises in mathematics. Various q-versions of this sequence are found, and their relations with Euler's pentagonal numbers theorem and Chebyshev polynomials are discussed. 
Boris A Kupershmidt