Maximum number of limit cycles for Abel equation having coefficients with linear trigonometric functions

This paper devotes to the study of the classical Abel equation $\frac{dx}{dt}=g(t)x^3+f(t)x^2$, where $g(t)$ and $f(t)$ are trigonometric polynomials of degree $m\ge 1$. We are interested in the problem that whether there is a uniform upper bound for the number of limit cycles of the equation with respect to $m$, which is known as the famous Smale-Pugh problem. In this work we generalize an idea from the recent paper (Yu, Chen and Liu, arXiv:$2304.13528$, $2023$) and give a new criterion to estimate the maximum multiplicity of limit cycles of the above Abel equations. By virtue of this criterion and the previous results given by {Á}lvarez et al. and Bravo et al., we completely solve the simplest case of the Smale-Pugh problem, i.e., the case when $g(t)$ and $f(t)$ are linear trigonometric, and obtain that the maximum number of limit cycles, is three.