Grassmann graph eigenvalue monotonicity and exceptional minimum conjecture

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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\rectd−10\rect i\rect d-1. (ii) If (n,q)=(2d,2)(n,q)=(2d,2), then Gj(d−j)G_j(d-j) is negative for (d,j)=(5,3)(d,j)=(5,3) and when d\rect6d\rect 6, 2\rectj\rectd−22\rect j\rect d-2, and Gj(d−j)G_j(d-j) is the smallest among the Gj(i)G_j(i) when d\rect6d\rect 6, 3\rectj\rectd−23\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\rectd−57\rect j\rect d-5 and remains open for the listed exceptional values of jj.

References

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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