Almost Differentially Nondegenerate Nijenhuis Operators

The paper is devoted to the study of Nijenhuis operators of arbitrary dimension $n$ in a neighborhood of a point at which the first $n-1$ coefficients of the characteristic polynomial are functionally independent, and the last coefficient (the determinant of the operator) is an arbitrary function. We prove a theorem on the general form of such Nijenhuis operators, and also obtain their complete description for the case in which the determinant has a nondegenerate singularity.