text: 178,88,1171326547569106944,1153187867897860096,Nikita Nikolaev,New Paper: I construct Levelt filtration for singularly perturbed linear systems of #ODE (in rank 2 at a regular singular point) maintaining very tight #asymptotic control by upper-triangularising such system in a singular #perturbation families. <LINK> I put this paper out whilst attending the @NCCRSwissMAP General Meeting in Villars-sur-Ollon. Hit submit with the Alps in my view.,https://arxiv.org/abs/1909.04011,"We study singularly perturbed linear systems of rank two of ordinary differential equations of the form $\hbar x\partial_x \psi (x, \hbar) + A (x, \hbar) \psi (x, \hbar) = 0$, with a regular singularity at $x = 0$, and with a fixed asymptotic regularity in the perturbation parameter $\hbar$ of Gevrey type in a fixed sector. We show that such systems can be put into an upper-triangular form by means of holomorphic gauge transformations which are also Gevrey in the perturbation parameter $\hbar$ in the same sector. We use this result to construct a family in $\hbar$ of Levelt filtrations which specialise to the usual Levelt filtration for every fixed nonzero value of $\hbar$; this family of filtrations recovers in the $\hbar \to 0$ limit the eigen-decomposition for the $\hbar$-leading-order of the matrix $A (x, \hbar)$, and also recovers in the $x \to 0$ limit the eigen-decomposition of the residue matrix $A (0, \hbar)$. ","Triangularisation of Singularly Perturbed Logarithmic Differential