Complete Beelen-Datta-Ghorpade conjecture for projective point counts

From papers

Let er(d,m)e_r(d,m) be the maximal number of Fq\mathbb{F}_q-rational points of a projective algebraic set defined by rr linearly independent homogeneous degree-dd polynomials in m+1m+1 variables. Let Hj(d1,ml+1)H_j(d-1,m-l+1) be the affine point-counting quantity defined from the lexicographically ordered exponent vectors, and let πk=Pk(Fq)\pi_k=|\mathbb{P}^k(\mathbb{F}_q)|. Complete Beelen-Datta-Ghorpade conjecture. Suppose that m,d1m,d\geq 1, 1r(m+dd)1\leq r\leq\binom{m+d}{d}, and qd+1q\geq d+1. Let ll be the unique integer with 1lm+11\leq l\leq m+1 such that

(m+dd)(m+d+1ld)<r(m+dd)(m+dld).\binom{m+d}{d}-\binom{m+d+1-l}{d}<r\leq\binom{m+d}{d}-\binom{m+d-l}{d}.

Set

j=r(m+dd)+(m+d+1ld),j=r-\binom{m+d}{d}+\binom{m+d+1-l}{d},

so that 0<j(m+dld1)0<j\leq\binom{m+d-l}{d-1}. Then

er(d,m)=Hj(d1,ml+1)+πml.e_r(d,m)=H_j(d-1,m-l+1)+\pi_{m-l}.

This extends the incomplete Datta-Ghorpade formula from a restricted range of rr to all admissible rr. The source attributes the extension to Beelen, Datta, and Ghorpade and does not state a resolution, so the conjecture remains open here.

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Sources & referencesView supporting material

Primary source

Deepesh Singhal and Yuxin Lin, “On a conjecture of Beelen, Datta and Ghorpade for the number of points of varieties over finite fields”, arXiv:2311.07702 (2025).

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