S = solveSchubertProblem(P,k,n)
Represent a Schubert variety in the Grassmannian $Gr(k,n)$ by a condition $c$ either a partition or a bracket (see partition2bracket for details) and a flag $F$ (given as an $n \times n$ matrix). The codimension of the Schubert variety is $|c|$. A Schubert problem is a list of Schubert varieties, whose codimension add up to $k(n-k)$, which is the dimension of the Grassmannian.
The function solveSchubertProblem solves the given instance of the Schubert problem by the Littlewood-Richardson homotopy. This algorithm uses homotopy continuation to track solutions of a simpler problem to a general problem according to the specializations of the geometric Littlewood-Richardson.
This algorithm is described in the paper: Leykin, Martin del Campo, Sottile, Vakil, Verschelde "Numerical Schubert Calculus via the Littlewood-Richardson homotopy algorithm". Math. Comp., 90 (2021), 1407-1433. https://arxiv.org/abs/1802.00984.
For instance, consider the Schubert problem {2,1}$^3$ in $Gr(3,6)$, which has two solutions
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Its solutions to an instance given by random flags
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The Schubert conditions must be either all partitions or all brackets.
The object solveSchubertProblem is a method function with options.