s = LRrule(n,M)
LRrule uses the geometric Littlewood-Richardson rule to compute a product in the Chow ring of the Grassmannian. This writes a product of brackets as a formal sum of brackets, which represents an intersection of Schubert varieties as a formal sum of Schubert varieties. When the input matrix M is a Schubert problem, this gives the number of solutions to that Schubert problem. The command LRnumber calls LRrule and extracts the number of solutions.
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The output: [ 3 6 7 ]^3*[ 3 5 7 ]^2 = +10[1 2 3] means that the Schubert problem [ 3 6 7 ]^3*[ 3 5 7 ]^2 in multiplicative form has 10 solution 3-planes. That is, there are 10 3-planes that satisfy three Schubert conditions given by the bracket [3, 6, 7] and two conditions given by the bracket [3, 5, 7].
More generally, this computes a product in the Chow ring:
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The Littlewood-Richardson homotopy algorithm requires a Schubert problems (sum of codimensions equals the dimension of the Grassmannian).
The object LRrule is a method function with options.