Grassmann graph eigenvalue monotonicity and exceptional minimum conjecture

Let VV be a vector space of dimension nn over Fq\mathbb{F}_q, with n\rect2dn\rect 2d. The vertices of the Grassmann scheme Gq(n,d)G_q(n,d) are the dd-dimensional subspaces of VV, and Gj(i)G_j(i) denotes the eigenvalue of the distance-jj graph indexed by ii, for 0\recti,j\rectd0\rect i,j\rect d.

Grassmann eigenvalue conjecture. (i) If (n,q)eq(2d,2)(n,q) eq (2d,2), then

Gj(i+1)<Gj(i)|G_j(i+1)| < |G_j(i)|

where 0\recti\rectd10\rect i\rect d-1. (ii) If (n,q)=(2d,2)(n,q)=(2d,2), then Gj(dj)G_j(d-j) is negative for (d,j)=(5,3)(d,j)=(5,3) and when d\rect6d\rect 6, 2\rectj\rectd22\rect j\rect d-2, and Gj(dj)G_j(d-j) is the smallest among the Gj(i)G_j(i) when d\rect6d\rect 6, 3\rectj\rectd23\rect j\rect d-2.

Part (i) is proved for q\rect3q\rect 3 in the paper, while part (ii) is known for 7\rectj\rectd57\rect j\rect d-5 and remains open for the listed exceptional values of jj.

Sources & referencesView supporting material

Primary source

Sebastian M. Cioabă and Himanshu Gupta, “On the eigenvalues of Grassmann graphs, Bilinear forms graphs and Hermitian forms graphs”, arXiv:2102.10155 (2021).

Additional references

2 papers in this index state this conjecture (2018–2021). The statement above is taken from the most recent of them; the others are arXiv:1801.06034.

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