“…It also undescores the need to find alternative perturbations using other "small parameters," in particular, our calculations for the λSABR PDE are less tedious. See also [49,43,41] for related calculations for the small time asymptotics. Going further back in time, one needs to mention, among many other contributions, the works of Henry-Labordere [35,37,36] who used Riemannian geometry heat kernel approximations, the works of Gatheral and his collaborators who used heat kernel asymptotics to study the implied volatility, and the works of Lesniewski and his collaborators [31,32,33], who introduced and studied the SABR model, the work of Fouque, Papanicolaou, Sircar, and Solna [24,23,25], who studied stochastic volatility models and singular perturbation techniques in option pricing, see also [22].…”