  The relations in the tautological ring of the moduli space M_g of nonsingular
curves conjectured by Faber-Zagier in 2000 and extended to the moduli space of
stable pointed curves by Pixton in 2012 are based upon two hypergeometric
series A and B. The question of the geometric origins of these series has been
solved in at least two ways (via the Frobenius structures associated to 3-spin
curves and to CP1). The series A and B also appear in the study of descendent
integration on the moduli spaces of open and closed curves. We survey here the
various occurrences of A and B starting from their appearance in the asymptotic
expansion of the Airy function (calculated by Stokes in the 19th century).
Several open questions are proposed.
